ACCOUNT OF. THERMOMETERS. 2l5 



. receiver of an air-pump or in the torricelian vacuum : For, w;e 

 can never form a perfect vacuum with our machines, and even 

 in the tube of the barometer mercury always exifts. in the 

 form of vapours. Thefe vapours, though very rare, ilill con- 

 lain a large quantity of caloric, which the electricity muft dis- 

 engage in its paflage by com predion ; but the inllantaneous 

 augmentation of electricity which remits, cannot become fen- 

 iible on account of the little denfity of the medium ; but this 

 increafe is perceivable in denfer air, as we lee in the inftru- 

 ipent called Kinnerlley's thermometer. N 



The coniiderations which I have here made, appear to me Conclufion. 

 to point out with fome probability, that the phenomenon 

 called the eleclric fpark, is owing to the light difengaged from 

 the air by compreffion during the paflage of the electricity ; 

 fo that this phenpmenon is purely mechanical, and not at all 

 eleclric in itfelf. This is the notion which I fubmit to the 

 judgment of philosophers: if it be true, it muft lend consi- 

 derably to diminifti the number of hypothefes which have al- 

 ready been made, or may be made on the nature of electri- 

 city. For this reafon it is that I have offered it to their con- 

 sideration, requeuing that it may not be be thought that I 

 contider it as of greater importance than their deliberate exa r 

 mmation may beftow upon it. 



XIV. 



Account of Thermometers for regijtcring (he highejl and lowefi 



Temperatures in the Abfence of the Obfrver. % F, A. 



- 



To Mr. NICHOLSQN. *<mtih 



SIR, /raw 



XvilANY contrivances have been propofep! and adopted for Thermometers 

 registering all the ftations of the thermometer and barometer, "J^^" 

 by means of a float or other equivalent inftrument carrying a the weather.* 

 pencil, which marks its fjtuation on a furface gradually moved 

 along by means of a clock. Thefe, of which meteorologies 

 Jcnow the value, are nevertheless expend ve, and require a 

 degree of care and management futlicient to render Ampler 

 (^.v contrivances 



