IMPROVE- MENTS IN INDICATORS FOR ASCERTAINING THE POWER OF STEAM AND OTHER ENGINES
Year: 1879

page_text: LETTERS PATENT to Edward Thomas Darke, of No. 2, Brecknock Crescent,
Camden Road, in the County of Middlesex, for the Invention of "IMPROVE-
MENTS IN INDICATORS FOR ASCERTAINING THE POWER OF STEAM AND OTHER
ENGINES."
Sealed the 30th January 1880, and dated the 28th November 1879.
PROVISIONAL SPECIFICATION left by the said Edward Thomas Darke at
the Office of the Commissioners of Patents on the 28th November 1879.
EDWARD THOMAS DARKE, of No. 2, Brecknock Crescent, Camden Road, in the
County of Middlesex. "IMPROVEMENTS IN INDICATORS FOR ASCERTAINING THE
POWER OF STEAM AND OTHER ENGINES."
This Invention has for its object improvements in indicators for ascertaining the
power of steam and other engines.
The indicators now most commonly in use consist, as is well known, of a small
cylinder with a piston actuating levers and links, carrying a pencil in a to and fro
course parallel to that in which the piston moves, but through a greater space.
This pencil is caused to mark upon paper carried on the surface of a cylinder which
receives a reciprocating movement in unison with the piston of the engine which
has to be tested.
With such indicators great difficulty is experienced in obtaining accurate
diagrams from engines which run at very high speeds, and this arises from the
inomentum of the parts of the indicator interfering seriously with its indications
when the working speed is great. To obviate this I reduce very greatly the
dimensions of the cylinder and piston.
I reduce the spring to correspond with the lessened area of the piston and the
shortened stroke, and in short I effect a reduction in the size and weight of all the
parts.
I also reduce weight by improving the form of the spring; in place of making it
of round wire I make the steel wire rhomboidal in section, of a width equal to
about twice its thickness, so that the spring when fully compressed forms a complete
cylinder.
By adopting this form the greatest reduction in the weight of the parts may be
effected.
I also reduce the weight of the piston rod, and of the pencil arm, and the axis of
the pencil arm by drilling out the metal in the centre, thereby leaving them
hollow.
I also alter the arrangement for transmitting motion from the piston to the
pencil.

page_text: I joint to the head of the piston rod a socket or sleeve through which the pencil
arm is caused to pass. At one end of the arm is its axis, and on the further end
there is another slide which is moved by the arm up and down along a guide
parallel to the axis of the paper drum. On this slide the pencil is carried.
Or I may from the pencil arm of a flat strip of steel slotted to admit of the
longitudinal movement of the slides upon it.
In some cases I employ as a guide for the pencil slide a strip of metal with
a V form groove in it, and the back of the pencil enters this V groove, so that the
pencil is controlled both by its connection with the pencil slide and by the guide
in rear thereof. In this way steadiness is ensured. I connect the guide strip by
two springs to the moving support which carries the pencil to and from the paper,
and these springs serve to maintain a uniform pressure of the pencil upon the paper
whilst it traces the diagram upon it.
I clothe the cylinder of the indicator and the cylinder cover with wood or other
non-conducting material to prevent condensation and to facilitate handling.
I supply paper to the drum of the instrument from a roll kept within the drum.
I draw the paper, as required, out through a slot in the side of the drum, lap it
around the drum and secure it by means of the paper clips.
I also arrange upon the pencil guide or other fixed part of the instrument a train
of clockwork by which a second pencil is carried at a uniform speed in a direction
parallel to the axis of the paper drum. Once for each reciprocation of the drum
this pencil is caused to touch the paper. The speed of the engine at the time of
taking a diagram is then ascertained by counting the number of the marks thus
produced within a given distance, say, one inch measured on the surface of the
diagram paper.
For some descriptions of indicators it is convenient to carry the paper upon a
plane surface or table moved to and fro along straight guides by the engine.
I arrange on the back of the table a pair of guides in which a rod is able to slide
to and fro in a direction parallel to those in which the table itself is carried. The
rod is connected by links with a reciprocating part of the engine. A spring
attached to the table draws it along the guides until it is stopped by a collar on the
rod. The table then moves to and fro with the rod. To hold the table stationary
until it is desired to take a diagram I provide ratchet teeth in the edge of the table
and a pawl to drop between the teeth and lock the table at the end of its travel.
As soon as the the pawl is lifted the movement of the paper commences.
The indicator cylinder and piston are so placed as to nove the pencil along the
paper in a direction at right angles to the guides, along which the table moves.
I sometimes give a rifled or twisted form to the rod of the indicator piston. This
rod passes through a nut on the cylinder cover.
The nut has an arm upon it which by a link is connected with a similar arm on
an axis parallel to the cylinder, which axis consequently receives an angular
movement similar to that of the nut. The pencil arın is mounted on this axis, on
which, by a connection with the moving parts of the engine it is caused to slide.
The pencil under the influence of these two movements traces the diagram upon a
surface of paper carried within a concave. Or the pencil arm may be carried
directly by the nut and the concave will then receive from the moving parts of the
engine a reciprocating movement parallel to the axis of the indicator cylinder.
According to another arrangement I form teeth upon the rod of the indicator
piston to engage with a toothed are moving about its centre and carrying the
pencil arm. The pencil marks on paper carried within a concave, and the concave
receives a reciprocating movement from the engine in a direction at right angles to
that of the axis of the indicator cylinder.
In place of causing the piston rod and the arc to gear together with teeth a
similar motion may be obtained by connecting them by means of a pair of blade
Sprines

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Edward Thomas Darke in the Great Seal Patent Office on the
28th May 1880.
"
EDWARD THOMAS DARKE, of No. 2, Brecknock Crescent, Camden Road, in the
County of Middlesex. IMPROVEMENTS IN INDICATORS FOR ASCERTAINING THE
POWER OF STEAM AND OTHER ENGINES."
This Invention has for its object improvements in indicators for ascertaining the
power of steam and other engines.
The indicators now most commonly in use consist as is well known of a small
cylinder with a piston actuating levers and links carrying a pencil in a to and fro
course parallel to that in which the piston moves, but through a greater space.
This pencil is caused to mark upon paper carried on the surface of a cylinder which
receives a reciprocating movement in unison with the piston of the engine which
has to be tested.
With such indicators great difficulty is experienced in obtaining accurate diagrams
from engines which run at very high speeds, and this arises from the momentum of
the parts of the indicator interfering seriously with its indications when the working
speed is great. To obviate this I reduce very greatly the length of the cylinder
and the stroke of the piston.
I reduce the spring to correspond with the shortened stroke of the piston, and
in short I effect a reduction in the size and weight of all the parts moved by the
piston.
I also reduce weight by improving the form of the spring.
I also reduce the weight of the piston rod and of the pencil arm, and the axis
of the pencil arm, by drilling out the metal in the centre, thereby leaving them
hollow.
I also alter the arrangement for transmitting motion from the piston to the
pencil.
I joint to the lead of the piston rod a socket or sleeve through which the pencil
arm is caused to pass. At one end of the arm is its axis, and on the further end
there is another slide which is moved by the arm up and down along a guide
parallel to the axis of the paper drum. On this slide the pencil is carried.
Or I may form the pencil arm of a flat strip of steel slotted to admit of the
longitudinal movement of the slides upon it.
Figure 1 is a side elevation partly in section of an indicator constructed according
to my Invention and adapted for taking diagrams from rapidly running engines,
such engines, for example, as are employed in steam launches running at several
hundred revolutions per minute; Figure 2 is an end elevation of the same indicator,
but with a portion of the pencil arm removed; Figure 3 is a plan; a is the indicator
cylinder and b is its piston. The piston, as here shown, is of half the usual area, and
the springs are so proportioned that the stroke of the piston is reduced in a like ratio.
The piece of which the cylinder a forms a part is provided with a screw union at a¹
by which connection is made with the cylinder of the engine; all is an arm on the
same piece carrying the paper drum c in the usual manner; d is the spring of the
indicator, and this I make by preference of bollow drawn steel wire. The spring is
provided in the usual way with socket pieces at either end which screw on to
nozzles prepared to receive them, the one upon the piston b and the other on the
underside of the cover e, which screws into the top of the piece a. The piston rodƒ
passes out through a bole in the centre of the cover e; this rod is made of steel
and it is bored out hollow in the manner indicated by the dotted lines in Figure 1
from the upper end as far down as the top of the piston. f¹ is a head which screws
into the upper end of the piston rod. It is bored through to lighten it and to
receive the stem of the swivelling forked piece f". This forked piece by two pivot

page_text: screws holds the slider g. h is the pencil arm; it is made from a steel wire flattened
at one end to give it flexibility and slotted that the pencil may pass through it;
the unflattened part of the pencil arm passes through the slider g by a hole which
it fits accurately, and this portion of the arm is drilled out so as to render it tubular,
as is indicated by dotted lines in Figure 1. is a cross head fixed upon the pencil
arm and the ends of this cross head receive the points of the pivot screws i, i.
These screws are carried by the bracket k. The bracket k by means of its handle
at k¹ can be turned around upon the cover e, upon which it is mounted. The cover
has a circular boss on its upper surface and the bracket k has a corresponding hole
through it into which the boss on the cover is received, and the bracket is then
secured in its place by fixing on the plate e¹ to the top of the cover by means of
tap screws, as is seen in Figures 1 and 3. The bracket k also has fixed to it the
thin plate ¹¹, in which there is a straight guide slot at 11 to guide the pencil. "
is the pencil slide; it consists of a thin piece of metal, the edges are doubled over so
as to cause them to embrace the slotted end of the pencil arm upon which h" is
able to slide easily and truly. The pencil slide has a socket upon it into which the
brass pin, serving as a pencil, fits tightly. The pin also passes through the guide
slot, and when the bracket k is in the position in which it is indicated in the
Drawings the point of the pencil rests against the paper lapped around the paper
drum c. By turning the bracket the pencil can be removed from contact with
the paper, or the pressure with which it bears against the paper can be adjusted so
as to be just sufficient to cause it to make a distinct trace upon the paper without
being such as to cause needless friction. The arrangements for imparting rotary
motion to and fro to the paper drum do not differ from those now in use.
For a
like reason the method of using the instrument need not be described. This indicator
will produce an acccurate diagram from a fast running engine which the indicators
now in use will not do. The advantage is derived partly from the reduction which
I have made in the size, weight, and length of stroke of the piston, partly from the
lightening of the parts moved by the piston which follows on forming them hollow
or tubular, and partly from the lightening of the pencil arm and the freedom of
movement which results from improved design.
In some cases I employ as a guide for the pencil slide or strip of metal with a
V form groove in it, and the back of the pencil enters this V groove so that the
pencil is controlled both by its connection with the pencil slide and by the guide
in rear thereof. In this way steadiness is ensured. I connect the guide strip to
the moving support which carries the pencil to and from the paper, and a spring or
springs serve to maintain an uniform pressure of the pencil upon the paper whilst
it traces the diagram upon it.
This arrangement for guiding and supporting the pencil is shown by the Figures
4, 5, and 6. Figure 4 is an elevation of the bracket or support & in its modified
form, and with the parts which are mounted upon it; Figure 5 is a plan of the
same; Figure 6 shows one of the parts separately. The guide plate k" is not fixed
to the bracket but is furnished with pivot screws, the points of which enter cavities
in the top and bottom of the post /* fixed into the bracket . The plate k¹ has a
V form groove in it, indicated by dotted lines in Figure 4; it does not extend
through the plate, and the hinder end of the pencil or marking pin enters this
groove. is a spring clamped to the pillar k*, and giving an elastic support to the
plate The pencil arm works in this arrangement between the paper drum c
and the plate k¹¹ in place of being on the outer side of the guide plate k, as in the
arrangement shown by the Figures 1, 2, and 3.
Where the greatest possible lightness and freedom from friction is desired I
sometimes dispense with the pencil slide and pencil with its guide plate. I then
fix the metallic pencil or marking pin to the end of the pencil arm in such manner
that the side of the pin may come into contact with the paper on the paper drum,
the pin resting tangentially against the drum and marking with its side. An
objection to this arrangement however is that the line traced on the paper is apt
to be broad, and unless the instrument be carefully used and with a light pressure

page_text: against the paper the position of the arm is liable to be affected by the rotation of
the drum.
In some cases I clothe the cylinder of the indicator and the cylinder cover with
wood or other non conducting material to prevent condensation and to facilitate
handling, but especially the latter. This improvement is illustrated by the Figures
7 and 8. Figure 7 is a vertical section; and Figure 8 is a plan. a* represents the
annular piece of wood with which the upper part of the cylinder and piece a is
clothed. Rose wood or mahogany is a suitable material. The wood is held in its
place by a metal ring screwed on to the top of the picce a, as the Drawing indicates.
The cover e also receives a ring of wood around it, and this is held in its place by a
metal disc e screwed on to the lower part of the boss on which the bracket k turns.
When the parts are thus clothed with wood no difficulty will be experienced in the
use of the instrument in consequence of its becoming heated, which at the present
time gives rise to much inconvenience.
Another part of my Invention relates to the construction of the paper drum, with
a view more conveniently to supply it from time to time with the paper upon
which the diagrams are taken.
I supply paper to the drum of the instrument from a roll kept within the drum.
I draw the paper as required out through a slot in the side of the drum, lap it
around the drum, and secure it by means of the paper clips.
Figures 9, 10, and 11 represent the paper drum and some of the parts in con-
nection therewith. Figure 9 is an elevation; Figure 10 is a vertical section; and
Figure 11 is a plan. In these Figures a is the fixed spindle on which the drum
rotates; b is the base or lower part of the drum, around which the string which
moves the drum is lapped in the usual manner; c is the upper part or body of the
drum, it has a slit a formed in it parallel to the axis and extending to the top;
d is a cover entering the part c at the top, it turns on the spindle a and is held
down by the nut e screwed on to the spindle. A stud upon the cover enters the
notch in the body c so that the cover can only be inserted in one position. ff
are the paper clips, they are hinged at the lower part of the drum, and at their
upper ends they are provided with right angle projections which enter notches in
the cover d and so the clips are held in place. These clips however are of the
ordinary description. The roll of diagram paper is placed within the body e around
the spindle a; its end is led out through the slit a'; the paper is lapped around
the drum and secured by the paper clips. With the object of recording the speed
of the engine upon the paper on which the diagram is taken I also arrange upon
the pencil guide or other fixed part of the instrument a time measurer, by which a
second pencil is carried at an uniform speed in a direction parallel to the axis of
the paper drum. Once for each reciprocation of the drum this pencil is caused to
touch the paper. The speed of the engine at the time of taking a diagram is then
ascertained by counting the number of the marks thus produced within a given
distance, say, one inch measured on the surface of the diagram paper.
The way in which I arrange this time measurer is indicated by the Figures 12,
13, and 14, and the Figures 15, 10, and 17 show some of the parts to a larger
scale. In these Figures, a is an arm carried out in a convenient direction from the
support of the paper drum. On it the cylinder b is fixed. The cylinder contains a
cupped leather piston, and the piston rod, which works through suitable guides, has
rack teeth upon it. The space in the cylinder beneath the piston is filled with
water or liquid, and when the piston is raised it draws liquid in from a small india
rubber supply bag, which is connected at a³ by a short length of flexible tube. In
applying this bag care is taken to prevent air entering or afterwards being drawn
into the apparatus. The base of the cylinder b is shown to a larger scale in
elevation at Figure 15, in plan at Figure 16, and in section at Figure 17. The
base of the cylinder contains a small valve c which when the piston is raised lifts
and admits liquid freely from the bag, but closes to prevent its return, so that the
return has to take place by the small bye pass d. This passage can be contracted
to any desired extent by the screw plug d', so that the descent of the piston (which

page_text: is caused by the action of a spring) can be made to occupy a longer or a shorter
time, as may be desired. e is a bracket fixed to a metal jacket surrounding the
cylinder b, and capable of turning about it. The bracket e carries the upper guide
for the piston rod, and also a stud axis on which the pencil arm is free to turn.
The pencil is carried on a slide, and is controlled by a straight slit in a guide plate,
as already described, in respect to Figures 1, 2, and 3; but in this case the guide
plate is a fixture upon the jacket of the cylinder. The pencil arm has a longitu-
dinal slot in it, and a pin on the piston rod engages with this slot, (so that the
pencil arm follows the movements of the piston. On the same stud axis with the
pencil arm there is a barrel g containing a coiled spring. One end of the spring is
attached to the stud axis, and the other end to the interior periphery of the spring
barrel. is a toothed sector fixed to the barrel g, its teeth engage with the rack
teeth formed upon the piston rod. It is this spring which gives to the piston a
constant tendancy to descend in the cylinder. The paper drum i has in this case
a ridge formed upon it at in such a position that this ridge never passes beneath
the indicator pencil actuated by the steam piston. At each reciprocation of the
paper drum, however, the ridge passes the pencil of the time measurer shown in
these Figures, so that a mark is produced by this pencil upon the summit of the
ridge; is a handle fixed to the spring barrel g and sector h. To prepare the time
measurer for use the handle k is pressed down so as to bring the piston to the top
of the cylinder, then by means of the same handle the time measurer is turned
round until the pencil is brought into position to mark on the ridge of the paper.
This is done either immediately before, during, or immediately after taking the
indicator diagram. If by adjusting the screw plug d', prior to the use of the
instrument, it has been set to a certain speed (say, so that the piston takes a
quarter of a minute to run from end to end of its stroke), then all that is necessary
to ascertain the speed of the engine is to count the number of marks made by
the pencil whilst traversing over its course. Or the instrument may be used
without this previous accurate adjustment by causing the pencil of the time
measurer to be held in position to mark on the paper for a definite time, say, half
a minute or one minute. During the whole of this time the pencil of the time
measurer is caused, by its connection with the piston, to travel in a downward
direction, and a number of successive marks or ticks are produced in a column on
the paper by the side of the diagram. By counting these ticks the number of
strokes made by the engine per minute is ascertained.
In place of employing a spring controlled by a piston to obtain the slow move-
ment of the speed indicating pencil, clock work, or like mechanism, may be
employed, but the arrangement described is that which I prefer.
I sometimes give a rifled or twisted form to the rod of the indicator piston.
This rod passes through a nut on the cylinder cover.
The nut has an arm upon it which by a link is connected with a similar arın
on an axis parallel to the cylinder, which axis consequently receives an angular
movement similar to that of the nut. The pencil arm is mounted on this axis, on
which, by a connection with the moving parts of the engine, it is caused to slide.
The pencil, under the influence of these two movements, traces the diagram upon a
surface of paper carried within a concave. Or the pencil arm may be carried
directly by the nut, and the concave will then receive from the moving parts of
the engine a reciprocating movement parallel to the axis of the indicator
cylinder.
Figure 18 is an elevation, Figure 19 is a vertical section, and Figure 20 is a plan
of an indicator, in which the piston rod is twisted and works through a nut, and a
to and fro rotary motion is imparted from the nut to an axis, on which a pencil
arm is mounted in such manner as to partake of the rotary motion of the axis,
whilst at the same time it receives a motion parallel to the axis by a connection
with a moving part of the engine to which the indicator is applied.
a is the cylinder of the indicator; in this case I make the cylinder of the same
size as those of the indicators now in common use, for in this construction the

page_text: momentum of the moving parts has much less tendency to interfere with the proper
performance of the instrument. The piston is prevented from turning within the
cylinder by a groove formed within it, into which a tooth formed on the side of the
piston enters.
The spring employed is wound one half in either direction, in order that when
compressed or extended there may be no tendency to coil or uncoil. b is the rifled
or twisted piston rod, it passes through a corresponding passage formed in the nut c.
The nut c is held by the cover of the cylinder in such manner that it is free to
turn, and it has fixed upon it an arm d; this arm is connected by a link e with
another armf, which is fast upon an axis g. The axis g is held so as to be able to
turn very freely between pivot screws h, h, at its ends; these pivots are inserted
into the top and bottom plates i, i, which in turn are fixed to a bracket & formed
upon the side of a jacket surrounding the cylinder a. The axis g is square in
section, and the pencil arm is able to slide along it from end to end. Longitudinal
motion is imparted to the pencil arm by means of a fork m, which embraces a neck
formed upon the boss of the pencil arm. The fork m is attached to a dovetail
slide m¹, which is held in a corresponding groove in the face of the bracket / parallel
to the axis g. Two cords are attached to the slide m¹, one passing in an upward
and the other in a downward direction; these cords are led between the pulleys
n, n, and one of them is attached to a moving part of the engine from which the
diagram is to be taken, the other is connected with an india rubber or other spring.
The plates i, i, also carry another axis o, upon which arms p, p, carrying the
concave surface q are free to turn. The surface q is intended to support the paper
on which the diagram is drawn, and the paper is held against the inner surface of
the concave by the paper holding springs, r The paper is drawn out as required
fron the box 8. The springs r, r, are each attached at one end to a boss through
which the axis o passes, and the bosses are fixed upon the axis. The axis o also has
an arm o¹ fixed upon its upper end, and a pin can be passed through the outer end
of this arm into a hole prepared for it in the upper arm 22. When the pin is inserted
in this manner the springs 7, r, bear against the inner surface of the concave, or on
the paper which may have been previously inserted between the springs and the
concave, and so the paper is securely held. In taking the diagram the concave
with the paper upon it is held by the hand in position to be marked by the pencil.
The pencil is supported by a light spring to insure a sufficient uniformity in the
pressure of the pencil against the paper whilst the line is traced. The numerous
other Figures on the same Sheet with the Figures 18, 19, and 20, shew separate
parts of the mechanism, and they are lettered to correspond with the description
given above.
According to another arrangement I form teeth upon the rod of the indicator
piston to engage with a toothed arc, moving about its centre and carrying the
pencil arm. The pencil marks on paper carried within a concave, and the concave
receives a reciprocatory movement from the engine in a direction at right angles to
that of the axis of the indicator cylinder.
Figure 21 is a front elevation, Figure 22 is a side elevation, Figure 23 is a
portion of an elevation as seen from the other side, and Figure 24 is a plan of an
indicator, in which the pencil arm is actuated by teeth formed upon the piston rod,
and in which the pencil arm thus receiving a semi rotary motion is made to mark
upon a concave surface to which a to and fro rectilinear movement is imparted at
right angles to the plane in which the pencil arm moves. a, a, is the upper part
of the indicator cylinder, and b is the piston rod; the piston rod is grooved so as
to operate as a rack to give motion to a toothed sector c in every position which
the piston rod may assume. The axis of the sector c is carried by a bracket fixed
to the flange a¹ formed upon the cylinder. The axis turns freely between pivots,
and it carries the pencil arm d. The flange a' also has guide strips fixed upon it
between which the slide e is capable of moving in a direction perpendicular to the
plane in which the pencil arm moves; ex is an arm upon the slide e, and cords are
ttached to this arm. The cords pass either way to the guide pulleys f. f, and one

page_text: cord is attached to an india rubber spring whilst the other is connected with a
moving part of the engine from which the diagram is required to be taken. The
axes of the pulleys e, e, are carried by blocks, which are fixtures upon the flange a¹;
c', e¹, are curved standards fixed upon the slide e, and e¹¹ is a concave plate of thin
metal jointed at its lower end to the slide. The paper to receive the diagram is
placed between the standards e¹ and the concave e¹¹, and then these parts are closed
together and held by a spring catch.
In place of causing the piston rod and the arc to gear together with teeth a
similar motion may be obtained by connecting them by means of a pair of blade
springs.
Having thus described the nature of my said Invention and the manner of per-
forming the same, I would have it understood that I claim as my improvements in
indicators for ascertaining the power of steam and other engines,―
First. The construction of indicators, having the parts actuated by the piston, of
the proportions indicated by Figures 1, 2, and 3, so that the said indicators may be
capable of producing diagrams from high speed engines.
Second. Forming parts of indicators taking motion from the piston, tubular or
hollow, as described in respect to Figures 1, 2, and 3 of the Drawings.
Third. Forming the pencil arm with a flat slotted end, and arranging thereon a
pencil slide carrying a pencil working through a guide slot, as described in respect
to Figures 1, 2, and 3 of the Drawings.
Fourth. The method of guiding and supporting the pencil indicated by the
Figures 4, 5, and 6.
Fifth. The arrangement, shown by the Figures 7 and 8, to facilitate handling in
the use of the instrument and to prevent condensation.
Sixth. The paper drum adapted to receive the roll of paper within it and with
the opening at the side, such drum being combined with paper clips on each side
of the opening for fixing the paper around the drum.
Seventh. The combination with an indicator of a second or speed indicating
pencil marking on the paper at each oscillation of the paper drum and receiving
a progressive movement, substantially as described in respect to Figures 12, 13, 14,
15, 16, and 17.
Eighth. The construction of indicators with a twisted or rifled piston rod passing
through, and giving rotary motion to a nut which in turn is caused to impart an
angular motion to an arm carrying a pencil, marking upon a concave surface, as
described in respect to Figures 18, 19, and 20.
Ninth. The construction of indicators with a rack piston rod actuating a toothed
sector on an axis carrying a pencil arm and pencil, such pencil marking upon a
concave surface to which motion is imparted in a direction perpendicular to the
plane in which the pencil moves by connection with a moving part of the engine
from which the diagram is taken.
Tenth. The indicators for ascertaining the power of steam and other engines,
described by this Specification and substantially as represented by the annexed
Drawings.
In witness whereof, I, the said Edward Thomas Darke, have hereunto set my
hand and seal, this Twenty seventh day of May, in the year of our Lord
.