62 Defeription of a new Thermometer. 
Royal Society of London, is made to fhew its greateft rife 
or fall from one period of obfervation to another. This is 
done by means of two {mall pieces of black glafs, which 
float on two different furfaces of mercury, within two glafs 
tubes hermetically fealed. Thefe floats, when raifed to their 
greateft height, adhere to the fides of the tube, by means 
of a {pring of glafs, and become ftationary, although the mer- 
cury falls. After the obferver has noted the temperature, 
he, by a magnet held in his hand, draws down the float to 
the furface of the mercury, in confequence of a {mall bit of 
fteel wire being inclofed in the float, and the inftrument is 
prepared for another obfervation. This is an ingenious in- 
vention, but requires too delicate workmanfhip to be fit for 
common ufe: befides, it cannot be made to record the de- 
grees of heat at intermedtate periods. 
The thermometer lately invented by Dr. Rutherford, of 
Balilifh, and defcribed in the 3d vol. of the Edinburgh 
Tranfactions,, is alfo an ingenious contrivance, but has the 
fame defect of marking only the extreme points to which 
the liquor has rifen or fallen in two feparate glafs tubes. 
Some years ago it occurred to me, that an air thermometer 
might be ufed for the purpofes required ; and accordingly 
I read to this fociety a defcription of the infirument. But 
having formed another inftrument of a more fimple con- 
ftruction, I beg leave to give a defeription of it. 
AB, fig. 1. (Plate I.) isa tube about 14 inches long, and. 
3 of an inch cahbre, of thin glafs, fealed or clofed “at top, 
To the bottom, which is bent upwards, there is jomed a 
glafs tube 7 inches long, and -4,ths of an inch calibre, open 
at top.. The tube AB is filled with alcohol, and from 
B to E is filled with mercury.—As liquids are incompreffible 
by weight in any perceptible degree, neither the alcohol nor 
mercury will be affected by the weight of the atmofphere. _ 
FD 1s a {cale of brafs or ivory, about 6; inches long, di- 
vided in the ufual way. E is a {mall conical piece of ivory 
or glafs, of a proper weight, made to float on the furface of . 
the 
