thermometrieal barometer for measuring altitudes . i 87 
taming an excess of mercury, boil the thermometer in water, 
and if the instrument be wanted for the measurement of a 
height of 5000 feet, let the whole cool down to 300 ; if for 
10,000 feet let it cool to 190, drawing the mercury into the 
thread: and at that point hastily tilt off the mercury remain- 
ing in F, which may then be sealed while the whole is kept 
hot. On boiling it up again, the excess of mercury which rises 
into F will be detached from the thread by a gentle tap or 
two on the side with the nail, and will remain in the cap F for 
use when required. 
For mounting the thermometer, GH, Fig, 2. is a circular plate, 
one inch in diameter, with a hollow half cylinder K rivetted 
firmly through it, of sufficient size to admit the lower tube of 
the thermometer to be bound firmly to it, so that there be no 
shake, and no reliance for steadiness on the more tender part 
of the tube above. The hole L fits the tube pretty closely. 
Fig. 3. is a second circular plate 1,5 in diameter with two 
screw collars 1,15 diameter of the same external thread, and 
a hole M in its centre for the tube of the thermometer to pass 
through it. The holes L and M being opened conically in 
opposite directions, allow a little fine tow to be wound round 
the tube, and when the two plates are fixed together by 
screws passing through them, they close the tow round the 
tube, both to steady it and to prevent the escape of steam. 
Fig. 4, is the scale, 5 inches long, 0,9 broad ; between the 
two standards N O a length of 4,15 inches is divided into 100 
parts, and by the Vernier into 1000, giving 241 parts to the 
inch ; this was accidental, being occasioned originally by the 
thread of my adjusting screw, which assisted me in making 
the divisions. The scale is fixed down to the upper plate, 
