50 



meter across the face, and one inch and three-quarters 

 in thickness. The pressure of the atmosphere is indi- 

 cated by a hand pointing to a scale, which is graduated 

 to correspond with the common barometer : thermome- 

 ters are placed on the face, one of which is essential. 



Fig. 2. 



Fig. 2 represents the internal construction as seen 

 when the face is removed, but with the hand still at- 

 tached, a is a flat circular box made of some white 

 metal, exhausted of air through the short tube b, which 

 is subsequently made air-tight by soldering : the upper 

 and lower surfaces of the box are corrugated in concen- 

 tric circles, which gives it greater elasticity; and the 

 box is fixed to the bottom of a metallic case, which in- 

 closes the mechanism of the whole instrument. In the 



