172 



WELLS'S NATUKAL PHILOSOPHY. 



upon the open surflice, is raised or lowered in the 

 tube, moving the index over a dial-plate, upon which 

 the various changes of the weather are lettered. 



Fig. 160 represents the internal structure of the 

 wheel-barometer, and Fig. 161 its external appear- 

 ance, or casing, with a thermometer attached. 

 _ ., , A very curious barometer, called 



Aneroid Ba- the "Aneroid Barometer," has been 

 rometer. invented and brought into use withm 



the last f?w years. Fig. 162 resprcsents its ap- 

 pearance and construction. Its action is dependent 

 en the effect produced by atmospheric pressure on a 

 Fig. 162. metal box, from 



which the air 

 has been ex- 

 hausted. In the 

 interior of the 

 box is a circu- 

 lar spring of 

 metal, fastened 

 at one extremi- 

 ty to the sides 

 of tlie box, and 

 attached at the 

 otlier extremity 

 by a suitable ar- 

 rangement to a 

 pointer, which 

 moves over a 

 dial-plate, or 

 scale. The in- 

 terior of the box being deprived of air, the atmospheric pressure upon tho 

 external surfaces of the metal sides is very great, and as the pressure varies, 

 these surfaces will be elevated and depressed to a sliglit degree. Tliis motion 

 is communicated to the spring in the interior, and from thence to the pointer, 

 which, moving upon the dial, tlius indicates the changes in the weather, or 

 the variation in the pressure of tho atmosphere. 



Water, or some other liquid than mercury, may be used for 

 peculiarities of filling the tube of a barometer. But as water is 13-J^ times 

 lighter than mercury, the height of the column in the water- 

 barometer supported by atmospheric pressure, will be 13^- times 

 greater than that of mercury, or about 34 feet high ; and a change wliieh 

 would produce a variation of a tenth of an inch in a column of mercury, would 

 produce a variation of an inch and a third in tlie column of water. Tho 

 Water-barometer is rarely used, for various reasons, one of whioli is, that a 

 barometer 34 feet high ia unwieldy and diJScult to transport. 



tlie water-ba- 

 rometer ? 



