45, CORNHILL, E.G., AND 122, REGENT STREET, W., LONDON. 2T 



tracing, which are half an inch apart, represent one-tenth of an inch change in 

 the barometer. The vertical lines are hour lines, and being nearly three- 

 quarters of an inch apart, it will be seen that the smallest appreciable change 

 in the barometer, and the time of its occurrence, are recorded. The barometer 

 in this instrument is similar to Mr. McNeild's " Long-Range Barometer," 

 described page 16. Constructed to order 280 to 300 



THE ANEROID BAROMETER. 



27. The Aneroid Barometer. The extremely ingenious instrument 

 called the Aneroid, is no less remarkable for the scientific principles of its con- 

 struction and action, than for the nicety of its mechanism. As its name implies, 

 it is constructed " without fluid." It was invented by M. Vidi, of Paris. In 

 the general form in which it is made it consists of a brass cylindrical case 

 about five inches in diameter and two inches deep, faced with a dial 

 graduated and marked similarly to the dial-plate of a " wheel-barometer," upon 

 which the index or pointer shows the atmospheric pressure in inches and 

 decimals in accordance with the mercurial barometer. Within the case, is 

 placed a flat metal box made of German Silver, generally not more than half an 

 inch deep and about two inches or a little more in diameter, from which nearly 

 all the air is exhausted. The top and bottom of this box is corrugated in con- 

 centric circles, so as to yield inwardly to external pressure, and return when it 

 is removed. The pressure of the atmosphere continually changes, and with this 

 varying pressure, the top and bottom of the box approach to and recede from 

 each other by a small quantity ; but the bottom being fixed to the base, nearly 

 all this motion takes place on the top. The top of the box is elastic, and rises 

 and falls according as the compressing force lessens or increases. To the eye 

 these expansions and contractions are not perceptible, so small is the motion. 

 But they are rendered very evident by a delicate mechanical arrangement, 

 communicating with a system of levers ; and, by the intervention of a piece of 

 watch-chain and a fine spring passing round the arbour, turning the index to 

 the right or left, according as the external pressure increases or decreases. 

 Thus, when by increase of pressure the vacuum box is compressed, the 

 mechanism transfers the movement to the index, and it moves to the right ; 

 when the vacuum box expands under diminished pressure, the motion is 

 reversed, and the index moves to the left. As the index traverses the dial, 

 it shows upon the scale the pressure corresponding with a good mercurial 

 barometer. 



The Aneroid being placed under the receiver of an air pump the scale is laid 

 off to correspond with a Mercurial Barometer Gauge, and afterwards compared 

 and corrected by a Standard instrument. 



