SCIENCE OF COMMON THINGS. 



103 



Wheel-barometer. 



Use of barometer on land unreliable. 



meter (by showing the variations in 

 the weight of the air) indicates the 

 changes of the weather also. 



650 How is the common form of barometer, 

 called the wheel-barometer, constructed f 



The barometer consists of a bent 

 tube, filled with mercury, as repre- 

 sented in Fig. 27, the column be- 

 ing sustained by the pressure of the 

 atmosphere upon the surface of the 

 mercury in the shorter arm, the end 

 of which is open. A small float of 

 iron or glass rests upon the mercury 

 in the shorter arm of the tube, and 

 is suspended by a slender thread, 

 which is passed round a wheel car- 

 rying an index. As the level of the 

 mercury is altered, and the weight 

 raised or lowered in the tube, the 

 index moves; and as the divisions 

 on the circumference of the circles 

 within which it moves are much 

 amplified, very slight changes are 

 easily read off. 



Fig. 27 represents the internal structure of 

 the wheel-barometer, and Fig. 28 its external 

 appearance, or casing, with a thermometer 

 attached. Fjg ;8 



651 Why is the ordinary use of the barometer on the land extremely 

 limited and uncertain? 



The height of the mercury in the tube at any time 

 must depend partially upon the elevation of the place 

 of observation above the level of^ the sea ; and no correct 

 judgment can be formed relative to the density of the 

 atmosphere as affecting the state of the weather, with- 

 out reference to the situation of the instrument at the 

 time of making the observation. Therefore, no atten- 

 tion ought to be paid to the words, "fair, rain, change- 

 able" etc., frequently engraved on the plate of a baro- 

 meter, as they will be found no certain indications of 



