thermometrical barometer for measuring altitudes. 1 93 
be imagined. At the lowest station (having previously drawn 
into the tube the whole globule in F by expanding the mercury 
and connecting the thread at top) boil the thermometer; 
put a wooden peg into the steam vent, and by that forced 
expansion shake off a few parts at the top of the scale, so 
that the boiling point may be there accurately taken. Ascend 
till on boiling again the point is near the bottom of the scale, 
which will be with this particular instrument about 2200 feet. 
After an observation at this second station, adapt the ther- 
mometrical barometer to measuring in like manner another 
ascent to a third station, by drawing the globule in again, 
and expanding out the excess as before, so that the mercury 
may stand again at the top of the scale, and its point may be 
noted. Hence proceed to a fourth station, and so in like 
manner to others, getting the difference between the several 
stations, and consequently the aggregate height. The only 
correction requisite is for the specific gravity of air at diffe- 
rent temperatures given by General Roy, Phil. Trans. 67, 
770, for this a small thermometer is wanted, and will find 
room in the case among the folds of the linen tent. 
The experiments, which I have been able to make on alti- 
tudes, have been few, and they were made with a different 
instrument, which was unluckily broken. It had more sen- 
sibility than that which I have described, i° Fahrenheit being 
equal 10552 parts on my scale, or 2,3 inches. By a few obser- 
vations, such as the height of my house afforded, I reckoned 
552 parts equal to 530 feet in altitude. With that instru- 
ment, boiled on the counter of a bookseller's shop in Pater- 
noster-row, which I estimated between four and five feet 
Cc 2 
