New Form of Mountain or other Barometer. 237 
I apprehend it would be but accidental, if two barometers, made 
even by the same person but at different periods, or from any cause 
containing mercury of different lots, agreed in the specific grav- 
ity as nearly as this. 
Finally, not to make too long discussion of such a point, it is 
even rare to find two observers, in direct succession upon the 
same instrument, reading to a coincidence as close as the thou- 
sandth of an inch. I thought myself therefore justified in dis- 
carding a graduation which, as I before said, multiplies the refine- 
ment without increasing the certainty. 
Nevertheless, in limiting the tube-graduation to tenths of inches, 
I have not shut out the means of subdivision to thousandths in a 
ready and unexceptionable manner; whenever such subdivision - 
should be requisite. ‘These means consist in the application of 
a reading microscope or micrometer, whose position on the tube 
is held and regulated by a spring and clamp. The zero is ad- 
justed, first, to the now magnified image of the mercury surface : 
and then, by the motion of the screw, the space between said sur- 
face and the nearest division on the tube below, is measured in 
hundredths of inches by the comb of the micrometer, and thou- 
sandths on the head of the screw in a well-known manner. 
6. A piece of watch-spring, about three inches in length, is bent 
round at one end, so as to embrace three quarters of the circum- 
ference of the tube: the other end, which will then project from 
the tube, has merely a light triangular notch made in its upper 
edge to catch the loop of string or wire, which suspends the ther- 
mometer. In order to prevent any unsteadiness or wriggling 
motion, which would be otherwise likely in so narrow a strip, a 
piece of an inch length, from the same spring, is fastened with a 
single rivet at right angles to the former, so as to be vertical or 
nearly so when the clasp is made; or, as was done in the present 
instance, a cruciform piece is cut out of a wide clock-spring. 
7. Nothing more need be said of the float than that it is of ivo- 
ry, worked as thin and light as is consistent with its safety; and 
that the distance, between its under surface and the parallel fidu- 
cial edge of the rectangular notch seen in it, must be exactly equal 
with the space between the zero mark on the tube and that other 
line next above the zero mark, which has been spoken of already 
in $4. It is obvious that in such case, when the image of said 
fiducial edge seen by reflection on the tube, coincides with the 
