204 DESCRIPTION of a 



furfaces of mercury, within two glafs tubes hermetically fealed. 

 Thefe floats, when raifed to their greateft height, adhere to the 

 fide of the tube, by means of a fpring of glafs, and become fta- 

 tionary, although the mercury falls. After the obferver has 

 taken a note of the temperature, he, by a magnet held in ;iis 

 hand, draws down the float to the furface of the mercury, in con- 

 fequence of a fmall bit of fteel wire inclofed in the float, and the 

 inftrument is prepared for another obfervation. This is an in- 

 genious invention, but requires too delicate workmanfhip to be 

 fit for common ufe ; befides, it cannot be made to record the 

 degrees of heat at intermediate periods. The thermometer, 

 lately invented by Dr Rutherford of Balililh, and defcribed 

 in the 3d volume of the TrajifaSlions of this Society, is alfo an 

 ingenious contrivance, but has the fame defe<5l of marking on- 

 ly the extreme points, to which the Uquor has rifen or fallen, in 

 two feparate glafs tubes. 



Several years ago it occurred to me, that an air thermo- 

 meter might be ufed for the purpofes required, providing the 

 weight of the atmofphere could be excluded, or a counter-ba- 

 lance formed to it; and as the whole inftrument could be made to 

 rife and fall by the temperature of the atmofphere alone, it might 

 be adapted to a piece of clock-work, which would record the 

 degrees of heat at every inftant through the year : And accord- 

 ingly I read to this Society a defcription of the inftrument. 

 But having formed another inftrument, of a more Ample con- 

 ftrudlion, to anfwer the fame purpofe, I beg leave to give a de- 

 fcription of it. 



AB is a tube about 14 inches long, (PI. VI.) and three-fourths 

 of an inch caliber, of thin glafs, fealed or clofe at top To the bot- 

 tom, which is bent upwards, there is joined a glafs tube 7 inches 

 long, and four-tenths of an inch caliber, open at top. The tube 



AB 



