40 Browning's New Aneroid Barometer. 



the small portion of elastic air left in the chamber to lessen 

 the amount of collapse that has been produced. 



Here then we have a sensitive instrument, but its move- 

 ments of expansion and collapse are too small to be seen ; but 

 anybody who has handled a long-bladed pair of scissors can 

 understand how we may make a very minute change visible at a 

 glance. A slight motion of the handles of the scissors — which 

 are near the pivot, or fulcrum — induces a large amount of mo- 

 tion at the ends of the blades. If, therefore, the minute mo- 

 tion of the walls of our vacuum chamber can be communi- 

 cated to the short leg of a lever, which has a long leg on 

 the other side of the fulcrum, the extremity of the long leg 

 will move a great deal, while the short leg traverses a very 

 small space. 



It is not necessary that we should explain the details of 

 aneroid construction, if we have made the principle clear. 

 In good aneroids the chamber is formed of thin elastic cor- 

 rugated metal, and is usually about the size of a crown piece. 

 If such an instrument is carried up a high mountain, the 

 actual movement of the chamber walls will not, perhaps, ex- 

 ceed one hundredth of an inch, or the thickness of a sheet of 

 common writing paper. By the lever contrivance this is mul- 

 tiplied so as to carry an index hand over a considerable arc 

 of a circle : it may be of six or eight inches diameter. 



We have now arrived at a pretty accurate conception of an 

 aneroid, as usually made, whether it has the dimensions of a 

 stout silver watch, or is as big as a moderate sized clock. 



Let us now suppose that, instead of such instruments as 

 we have described, and which are portable, it is required to 

 make an extremely delicate stationary aneroid, capable of 

 responding to every fluctuation of atmospheric pressure, with 

 a sensitiveness not possible in instruments that are made to 

 withstand the shaking incidental to carrying them about, and 

 which shall indicate, and, if needs be, register, changes that 

 must be exhibited on a large scale if they are to be seen at all. 

 Mr. Browning set himself the difficult task of contriving an 

 aneroid to meet these wants, and the result is the exquisite 

 instrument represented in the annexed drawing. 



In the first place he did away with the spring. We have 

 seen that its function is to counterpoise a certain weight. 

 Now, although a spring will balance a given weight, it is ob- 

 vious that one weight may be made to balance another. Ac- 

 cordingly, Mr. Browning made his aneroid upon the principle 

 of a steelyard. At one end of a beam, suspended like that of 

 a fine chemical balance, so as to turn with the greatest nicety, he 

 fixes a weight (w) . The beam is suspended like a steelyard beam, 

 not from its centre but much nearer one end. The short end 



