IMPROVEMENTS IN BAROMETERS.
Year: 1870

page_text: LETTERS PATENT to Thomas Telford Macneill, of No. 1, Warwick
Street, Charing Cross, in the County of Middlesex, for the Inven-
tion of "IMPROVEMENTS IN BAROMETERS.'
"
Sealed the 13th June 1871, and dated the 15th December 1870.
PROVISIONAL SPECIFICATION left by the said Thomas Telford Macneill
at the Office of the Commissioners of Patents, with his Petition,
on the 15th December 1870.
I, THOMAS TELFORD MACNEILL, of No. 1, Warwick Street, Charing
Cross, in the County of Middlesex, do hereby declare the nature of the
said Invention for "IMPROVEMENTS IN BAROMETERS," to be as follows:-
My Invention has for its object the construction of a portable
barometer that admits of being made of a small size as compared with
ordinary barometers, and that shall always give very accurate readings.
According to my Invention in place of causing the mercury to be
forced by the atmospheric pressure up a closed exhausted tube as in
ordinary barometers, the barometer tube is open to the atmosphere,

page_text: and communicates together with a closed chamber containing air at
atmospheric density with a vessel containing mercury or other fluid. The
mercury is forced from such vessel simultaneously into the barometer tube
and into the air chamber, so that as by the entrance of the mercury the
air is compressed to a certain extent in the chamber the pressure thus
produced will force the mercury up the barometer tube, but only to a
height corresponding to the increase of pressure in the air chamber as
the ordinary atmospheric pressure therein is balanced by the atmospheric
pressure in the open barometer tube.
In taking observations with this instrument the mercury is for each
reading withdrawn from the air chamber, so as to cause this to be filled
with air at the actual atmospheric density, and the volume of such air
is for every reading compressed to exactly the same extent.
Thus it will be seen that if the atmospheric pressure at the time of
observation were equal to say thirty inches of mercury, and the air in
the chamber were compressed so as to occupy only about two-thirds of
its original volume, the mercury would stand in the barometer tube at
10 inches, whereas if the atmospheric pressure were, say, 21 inches, and
the same compression were then effected, the mercury in the tube would
stand at 7 inches, so that by forming on the barometer tube a propor-
tionately reduced scale the pressure of the atmosphere may thus be
correctly read off.
It will be evident that if the compression of the air were effected to
any extent beyond about one half its volume, an enlarged reading
instead of a reduced one would be obtained.
Variously modified arrangements of apparatus may be employed in
carrying my above-described Invention into effect.
Thus in the arrangement which I prefer to employ, the barometer
tube is made to pass down through the air chamber, both being open
at the lower end. The air chamber contains two diaphragms or con-
tractions, one at the mouth having an aperture somewhat larger than
the bottom of the barometer tube which passes into such aperture, and
the other at the exact point in the air chamber corresponding with the
proportion of the volume to be compressed, which second diaphragm
also has a small aperture through which in taking an observation the
mercury is just forced sufficiently to project up to a gauge mark or

page_text: point situated slightly beyond the surface of the diaphragm (or a guage
mark or point may be employed without the upper diaphragm) the
object of this being to insure the exact compression of the air to
precisely the same volume at each reading.
The mercury is contained in a bag inside a casing, which in travelling
can be removed from the barometer for convenience, and for use it is
securely screwed on to the lower end of the air chamber or on to a casing
enclosing the same. Or the mercury bag may be permanently attached
to the instrument. The mercury is forced up out of the bag into the
chamber by means of a screw made to act on the bag in any suitable
known manner. On forcing up the mercury it first enters the aperture
of the lower diaphragm, thereby closing the lower end of the barometer
tube, after which in rising in the space of the air chamber between the
two diaphragms it forces the air contained therein into the upper part
of the air chamber and thus compresses the air to that extent.
When no observations are being taken the mouth of the mercury
vessel may be closed by a screw stopper so as to prevent any escape of
the mercury.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said Thomas Telford Macneill in the Great Seal Patent
Office on the 14th June 1870.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, THOMAS
TELFORD MAONEILL, of No. 1, Warwick Street, Charing Cross, in the
County of Middlesex, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Fifteenth day of December, in the
year of our Lord One thousand eight hundred and seventy, in the thirty-
fourth year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said Thomas Telford Macneil, Her special
licence that I, the said Thomas Telford Macneil, my executors, ad-
ministrators, and assigns, or such others as I, the said Thomas Telford
Macneill, my executors, administrators, and assigns, should at any time
agree with, and no others, from time to time and at all times thereafter
during the term therein expressed, should and lawfully might make,

page_text: use, exercise, and vend, within the United Kingdom of Great Britain
and Ireland, the Channel Islands, and Isle of Man, an Invention for
"IMPROVEMENTS IN BAROMETERS," upon the condition (amongst others) that
I, the said Thomas Telford Macneill, my executors or administrators, by
an instrument in writing under my, or their, or one of their hands and
seals, should particularly describe and ascertain the nature of the said
Invention, and in what manner the same was to be performed, and
cause the same to be filed in the Great Seal Patent Office within six
calendar months next and immediately after the date of the said Letters
Patent.
NOW KNOW YE, that I, the said Thomas Telford Macneill, do hereby
declare the nature of the said Invention, and in what manner the same
is to be performed, to be particularly described and ascertained in and
by the following statement thereof, reference being had to the accom-
panying Sheet of Drawings and to the figures and letters marked thereon,
that is to say :-
My Invention has for its object the construction of a portable mercurial
or other fluid barometer that admits of being made of a small size as
compared with ordinary barometers, and that shall always give accurate
readings.
'
According to my Invention in place of causing the mercury to be
forced by the atmospheric pressure up a closed exhausted tube, as in
ordinary barometers, the barometer tube is opened to the atmosphere,
and communicates together with a closed chamber containing air at
atmospheric density with a vessel containing mercury or other fluid.
The mercury is forced from such vessel simultaneously into the barometer
tube, and into the air chamber, so that as by the entrance of the
mercury the air is compressed to a certain extent in the chamber the
pressure thus produced will force the mercury up the barometer tube,
but only to a height corresponding to the increase of pressure in the air
chamber, as the ordinary atmospheric pressure therein is balanced by
the atmospheric pressure in the open barometer tube.
In taking observations with this instrument the mercury is for each
reading withdrawn from the air chamber so as to cause this to be filled
with air at the actual atmospheric density, and the volume of such air is
for every reading compressed to exactly the same extent.

page_text: Thus it will be seen that if the atmospheric pressure at the time of
observation were equal to, say, thirty inches of mercury, and the air in
the chamber were compressed so as to occupy only about two-thirds of
its original volume, the mercury would stand in the barometer tube at
10 inches, whereas if the atmospheric pressure were, say, 21 inches, and
the same compression were then effected, the mercury in the tube would
stand at 7 inches, so that by forming on the barometer tube a propor-
tionally reduced scale the pressure of the atmosphere may thus be
correctly read off.
It will be evident that if the compression of the air were effected to
any extent beyond about one-half of its volume an enlarged reading
instead of a reduced one would be obtained.
Variously modified arrangements of apparatus may be employed in
carrying my above-described Invention into effect.
Thus in the arrangement which I prefer to employ the barometer tube
is made to pass down through the air chamber, both being open at the
lower end. The air chamber contains two diaphragms or contractions,
one at the mouth having an aperture somewhat larger than the bottom
of the barometer tube which passes into such aperture, and the other at
the exact point in the air chamber corresponding with the proportion of
the volume to be compressed, which second diaphragm also has a small
aperture through which in taking an observation the mercury is just
forced sufficiently to project up to a gauge mark or point situated
slightly beyond the surface of the diaphragm (or a gauge mark or point
may be employed without the upper diaphragm), the object of this
being to insure the exact compression of the air to precisely the same
volume at each reading.
The mercury is contained in a bag inside a casing which in travelling
can be removed from the barometer for convenience, and for use it is
securely screwed on to the lower end of the air chamber or on to a
casing enclosing the same. Or the mercury bag may be permanently
attached to the instrument. The mercury is forced up out of the bag
into the chamber by means of a screw made to act on the bag in any
suitable known manner. On forcing up the mercury it first enters the
aperture of the lower diaphragm, thereby closing the lower end of the
barometer tube, after which in rising in the space of the air chamber
between the two diaphragms it forces the air contained therein into
.

page_text: extent.
When no observations are being taken the mouth of the mercury
vessel may be closed by a screw stopper or cock so as to prevent any
escape of the mercury.
On the accompanying Drawing are shewn the arrangements of my
before-described improved barometer which I prefer to employ.
Figure 1 shews an elevation and Figure 2 shews a vertical section of
one arrangement.
The barometer tube A is open to the atmosphere at the upper end,
and passes centrally down through the air chamber B closed at the
upper end, as shewn, both parts being made of glass, and, by preference,
blown together at the point where the tube enters the air chamber. In
the air chamber a diaphragm C is fixed at a definite point in the height
thereof, having a lateral aperture c, and a second diaphragm D is fixed
near the lower open end of the air chamber having a central aperture d.
The barometer tube A which is reduced in diameter inside the air
chamber passes through the diaphragm C, and extends with its lower
open end a little into the aperture of the diaphragm D, which aperture
is somewhat larger than the tube A.
The air chamber and barometer tube are enclosed in a casing E, on the
lower end of which is formed a screw thread e by which the apparatus
is screwed on to the mercury vessel F; this vessel consists of a casing in
which is a mercury bag G acted upon by the screw H and piston I in a
somewhat similar manner to the mercury vessels of ordinary barometers.
At the upper end of the bag is a small aperture g by which it com-
municates with the parts A and B, and which can be closed by turning
the cock K. For taking an observation the cock K is turned so as to
open the aperture g, and the mercury is forced up out of the mercury
bag by the screw H, so as to pass through the aperture d in the
diaphragm D, and enter simultaneously the barometer tube A and the
air chamber B. The mercury is forced up until it entirely fills the
space in the air chamber between the diaphragms D and C, and passes
through the aperture c of the latter up to the gauge mark x, as shewn
in blue lines at Figure 1, the gauge mark being formed by the opposite
upper edges of two openings in the casing. By this means the air in
the air chamber, which was originally of the same density as the external

page_text: atmosphere is compressed to a definite extent, and is thus made to force
the mercury to a certain height up the barometer tube. This height
will, as before stated, vary in proportion to the variations in the original
density of the air in the air vessel at different points or times of observa-
tion, and consequently, by making a reduced scale L upon the casing E
the variations in the atmospheric density may be correctly read off from
the height of the mercury column upon such scale. M is a vernier, as
in ordinary barometers.
After an observation has been taken the mercury is withdrawn from
the tube A and air chamber B by screwing down the screw H, and the
cock K is turned so as to confine the mercury in the vessel F.
Figure 3 shews a vertical section of a slightly modified construction
of the before-described barometer. The barometer tube A and air
vessel B are here not enclosed in a casing, the scale being marked on
the tube itself, and the gauge mark to which the mercury is caused to
rise through the aperture c in the diaphragm C is formed of a pin x
fixed to the barometer tube.
•
In place of the stop-cock in the previous arrangement a screw plug N
is screwed into the opening of the mercury vessel F, which plug is
removed on unscrewing the mercury vessel when an observation is to
be taken.
Having thus described the nature of my Invention, and in what
manner the same is to be performed, I wish it to be understood that I
do not limit myself to the construction of my improved barometer shewn
on the accompanying Drawing, as it will be evident that the construc-
tion thereof may be variously modified without departing from the
essential feature of my Invention; but what I claim under the herein-
before in part recited Letters Patent is,-
First. Constructing barometers wherein the indication of atmospheric
pressure is obtained by forcing mercury into an air chamber containing
air at atmospheric density, in communication with which is an open
barometer tube in such manner that by the compression of air inside
the chamber the mercury is caused to rise in the tube to a height
corresponding to the degree of such compression, substantially as herein-
before set forth.
Second. The peculiar construction of barometers with open barometer
tube A, air vessel B, and diaphragms C and D acting in combination

page_text: with the mercury vessel F, substantially as herein-before described, with
reference to the accompanying Drawings.
In witness whereof, I, the said Thomas Telford Macneill, have here-
unto set my hand and seal, this Thirteenth day of June, in
the year of our Lord One thousand eight hundred and seventy-
one.
T. TELFORD MACNEILL. (L.S.)