46 



THERMOMETER AND PYROMETER. 



becomes as much lighter as the weight 

 of the mercury expelled from the tube, 



Fig. 44. 



and the weight at the top of the win- 

 dow, or of the ventilator, opens these 

 apertures which were kept shut by the 

 weight of the statical tube and ball. 

 On the other hand, when the cooling 

 of the air in the chamber causes the 

 contraction of the air included in the 

 ball, the pressure of the atmosphere 

 forces the mercury into the tube, which 

 thus becomes so much heavier ; and as 

 it descends, it drags with it the window 

 irame, or ventilator, attached to it. 



This simple and very ingenious con- 

 trivance is applicable to hot-houses, 

 ooms, and apartments of every de- 



1 D ' that are liable to consider- 

 changes of temperature; and it 



possesses considerable powers : for in a 

 tube two inches in diameter, every inch 

 in the rise or fall of the mercury is 

 equivalent to a moving power of about 

 one pound. It is liable to be slightly 

 altered also by changes in atmosphe- 

 ric pressure. 



Dr. Cummings's attention was drawn 

 to the importance of regulated tem- 

 perature in the treatment of disease, 

 whether in public institutions or in 

 private practice, and the contrivance 

 above noticed was the method by which 

 he endeavoured to obtain this important 

 object : but he soon perceived that the 

 principle was applicable to various mete- 

 orological purposes ; and the instrument 

 has, in his hands, undergone successive 

 modifications and improvements, until 

 it has become the basis of a thermo- 

 meter, hygrometer, and photometer, 

 capable of registering their own indi- 

 cations, by the aid of clock-work, at any 

 given time. Details of these different 

 contrivances would lead us into too 

 Fig. 45. 



