AN IMPROVED INSTRUMENT FOR MEASURING DISTANCES "
Year: 1883

page_text: LETTERS PATENT to John Tulloch Whish of Southsea in the County of
Hants Major in the Army for an Invention of "AN IMPROVED INSTRUMENT
FOR MEASURING DISTANCES
"
PROVISIONAL SPECIFICATION left by the said John Tulloch Whish at the
Office of the Commissioners of Patents on the 21st August 1883.
JOHN TULLOCH WHISH of Southsea in the County of Hants, Major in the
Army "AN IMPROVED INSTRUMENT FOR MEASURING DISTANCES
"
My invention relates to an instrument for measuring the distance of distant
objects situated at the apex of a triangle, the base of which is very small as
compared with the length of the sides. The apparatus and operation are most
simple when the triangle is nearly isosceles, each of the angles at its base being
nearly a right angle. In such cases, I employ apparatus of the following kind.
I provide a tripod stand carrying a horizontal plate, on which are fixed two pairs
of Y frames for carrying a telescope, like that of a theodilite, these two pairs of
frames being so placed relatively to one another that, when the telescope is placed
in the one pair, its axis is at right angles to the line of its axis when placed in the
other pair. I also mount on the telescope a straight bar, graduated in equal
divisions marked on both sides, the zero point being immediately over the centre of
the telescope, and fitted to turn on a vertical pin coincident with the zero division,
so that it can be placed either before or behind the telescope, and being marked
on both sides, can, in either of these positions, be read either from right or left
sides.
Having two such instruments, one situated at each end of the measured base, an
observer at either end placing his telescope on one pair of the Y frames directs it
towards the distant object, and by then transferring it to the other pair of
Y frames, reads off the division crossed by the axis of his telescope on the
graduated bar of the other instrument; the other observer does the same, and
from these readings and the length of the base the distance of the object can be
readily computed.

page_text: In the case where one angle of the base is exactly a right angle, the observer at
this point would read zero on the other bar, and therefore the reading made by the
other observer would alone determine the distance, but in all other cases two
readings are necessitated, and the sum of these is to be taken, if both bars aro in
the same position as regards their respective telescopes, that is to say, both directed
forwards or both backwards, the difference being taken if the two bars are
oppositely directed. Then, on dividing the square of the length of the base by
true reading thus found, we obtain the distance of the object. Ordinary theodolites
with graduated bars could obviously be employed as above described.
Theodolites so fitted could also be employed when the base angles of the triangle
differ considerably from right angles, the bars being placed at proper inclinations.
Thus the bar at one extremity of the base would have to be placed at an
inclination equal to double that base angle, and the observer at the other base
angle would read back the single angle increased by 180° to the division on the
bar which would have to be used as the divisor of the square of the base, in order
to determine the distance.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent and
of an Order of the Lord Chancellor filed by the said John Tulloch Whish in
the Patent Office on the 17th March 1884.
JOHN TULLOCH WHISH of Southsea, in the county of Hants, Major in the
Army "AN IMPROVED INSTRUMENT FOR MEASURING DISTANCES ง
My invention relates to an instrument for measuring the distance of distant
objects situated at the apex of a triangle, the base of which is very small as
compared with the length of the sides. The apparatus and operation are most
simple when the triangle is nearly isosceles, each of the angles at its base being
nearly a right angle. In such cases, I employ apparatus of the following kind.
I provide a tripod stand, carrying a horizontal plate, on which are fixed two pairs
of Y frames for carrying a telescope, like that of an ordinary theodolite, these two
pairs of frames being so placed relatively to one another that when the telescope is
placed in the one pair, its axis is at right angles to the line of its axis when placed
in the other pair. I also mount on the telescope a straight bar graduated in equal
divisions marked on both sides, the zero point being immediately over the centre
of the telescope, and fitted to turn on a vertical pin coincident with the zero
division, so that it can be placed either before or behind the telescope, and being
marked on both sides, can, in either of these positions, be read either from right or
left sides.
Having two such instruments, one situated at each end of the measured base, an
observer at either end, placing his telescope on one pair of the Y frames, directs it
towards the distant object, and by then transferring it to the other pair of Y frames
reads off the division crossed by the axis of his telescope on the graduated bar of
the other instrument; the other observer does the same, and from these readings
and the length of the base, the distance of the object can be readily computed.
In the case where one angle of the base is exactly a right angle, the observer at
this point would read zero on the other bar, and therefore the reading made by the
other observer would alone determine the distance, but in all other cases, two
readings are necessitated, and the sum of these is to be taken if both bars are in
the same position as regards their respective telescopes, that is to say, both
directed forwards or both backwards, the difference being taken if the two bars
are oppositely directed. Then, on dividing the square of the length of the base by
the true reading thus found, we obtain the distance of the object. Ordinary
theodilites with graduated bars could obviously be employed as above described.
Fig. 1 of the accompanying drawings is a side view of one form of instrument
which I employ for measuring distances as above described, fig 2 being a
perspective view shewing the frame with the telescope removed. This frame A,
which is mounted to turn on a tripod stand B, has four arms, each having a bearing
C for the telescope fitted in a vertical slide, so that is can be raised or lowered by
a screw D. These bearings are accurately adjusted so that the line of the axis of
the telescope E, when it lies in one pair of the bearings, is at right angles to the
line of the axis when it lies in the other pair. A graduated bar F is mounted on
a pivot G at the middle of the telescope, so that it can be turned either backward
as shewn or forward, as indicated by the dotted lines G³, in line with the axis of
the telescope, or nearly so. Fig. 3 shews a modified form of the instrument, which
is of advantage in saving the operation of shifting the telescope and in ensuring
accuracy. In this case, the telescope is made double, one tube E¹ crossing the
other E at right angles, each tube adjustable in inclination by screws H on a frame
K mounted on the tripod, with a clamp L and tangent screw M. Figs. 4, 5 and 6 are
diagrams explanatory of the use of the instrument for ascertaining the distance

page_text: of an object O from either extremity of a measured base A B. Three cases present
themselves. In fig 4 the triangle OA B is nearly isosceles, each of the angles at
A and B being less than a right angle. The observer at A directs his telescope in
one of its positions or one of the tubes of his double telescope to O, and then
observes through his telescope in its altered position or through its second tube a
point C on the graduated bar of B's telescope turned backwards thus ascertaining
the length of B C intercepted between B and the line A C which is perpendicular
to OA. In like manner the observer at B ascertains the length A D. Now, if
CE be drawn parallel to B D, the length A E will be very nearly the sum of BO
and A. D, and as the triangle CAE is similar to O A C, the sides are proportional,
that is to say EA: AC:: AC: AO, or the distanco A O can be computed by
dividing the square of the known base A B (which is very nearly equal to A C) by
the length A E which is the sum of the two observed lengths B C and A D as seen
on the graduated bars of the two telescopes. In the caso shewn by fig 5, whero
the angle at A happens to be a right angle, the observer at A will see the zero
point on the bar at B, and the measurement AD on the bar at A, as seen from B,
will be the divisor of the square A B., In the case shewn by fig. 6 where the angle
OAB happens to be greater than a right angle, the bar at B will have to be
directed forwards and D E which is nearly equal to BC will have to be substracted
from A D, the measurement on the graduated bar at A turned backwards, the
remainder A E being the divisor of the square of A B.
In cases where a base cannot be obtained, such as will make the triangle
approximately isosceles, the theodolite may be employed with the graduated bar
applied to it, as may be understood by the diagram fig. 7. In this case the
observer at A. ascertains by his theodolite the angle O A B and turns his graduated
bar AD to such a position that the angle B A D is equal to OA B. The observer
at B directs his theodolite at an angle F B D, the same as O A. B or B A D (that
is to say, he turns it from its direction BO through an angle OBD which is the
supplement of OA D) and observes the point D on the graduated bar at A. Now,
as the angle FBA is the sum of BOA and B A O, and F B D is equal to BA O,
therefore D BA is equal to BOA, and BAD being equal to OA B, the two
triangles BA D and OBA are similar, hence DA:AB::AB: A O, or AO is
equal to the square of A B divided by DA. Should the angle at A be obtuse as
shewn in fig. 8, the operation is the same, the graduated bar A D being turned to
make the angle B A D equal to OA B, and the theodolite at B directed so that
the angle F B D is also equal to OA. B.
Having thus described the nature of my invention and in what manner the same
is to be performed, I claim:-
In an instrument for measuring distances, the combination of a telescope by
which two directions at right angles to each other can be observed, with a
graduated bar, arranged and operated substantially as herein described
In Witness whereof I the said John Tulloch Whish have hereunto my hand
and seal this twenty eighth day of January in the year of our Lord One thousand
eight hundred and eighty-four
J. T. WHISH (L.S.)