22 NEGRETTI AND ZAMBRA, HOLBORN VIADUCT, E.G., 



The engraving ("fig. 23) represents the latest improved mechanism of an 

 aneroid. The outer casing and face of the instrument are removed, but the 

 index hand is left attached to the arbour. A is the corrugated vacuum box 



which has been exhausted of air through 

 the tube J, and hermetically sealed by 

 soldering. B is a powerful curved 

 spring, resting in gudgeons fixed on the 

 base-plate, and attached to a socket be- 

 hind, Fj in the top of the vacuum box. 

 A lever, (7, joined to the stoub edge of 

 the spring, is connected, by the bent 

 lever at I), with the chain, .E7, the other 

 end of which is coiled round, and fastened 

 FIG 23. to the arbour, F. As the box, A, is com- 



pressed by the weight of the atmosphere increasing, the spring, J9, is tightened, 

 the lever, (7, depressed, and the chain, E, uncoiled from F, which is thereby 

 turned so that the hand, H, moves to the right. In the meanwhile the spiraj 

 spring, G, coiled round F, and fixed at one extremity to the frame- work, and by 

 the other to F, is compressed. When, therefore, the pressure decreases, A 

 and B relax, by virtue of their elasticity ; E slackens, G unwinds, turning F, 

 which carries the index hand, H, to the left. Near / is shown an iron pillar, 

 cast as part of the stock of the spring, B. A screw works in this pillar through 

 the bottom of the plate, by means of which the spring, J?, may be so adjusted 

 to the box, A, as to set the index, H, to read on the scale in accordance with 

 the indications of a Mercurial Barometer. In the higher class of aneroid baro- 

 meters, the lever, C, is formed of a compound bar of brass and steel, so skilfully 

 arranged as to perfectly compensate for the effects of extreme variations of 

 temperature. 



The greatest perfection in Aneroids is now attained by having as perfect 

 and dry a Vacuum as possible. Compensation being obtained by the compound 

 metal bar previously mentioned. 



A Thermometer is sometimes attached to the Aneroid, as it is convenient 

 for indicating the present temperature of the air, but for accuracy and safety 

 from breakage, N. and Z. recommend the use of a separate Thermometer. 



Admiral FitzRoy, in his Barometer Manual, writes : " The Aneroid is quick 

 in showing the variation of atmospheric pressure ; and to the navigator who 

 knows the difficulty, at times, of using barometers, this instrument is a great 

 boon, for it can be placed anywhere, quite out of harm's way, and is not affected 

 by the ship's motion, although faithfully giving indication of increased or 

 diminished pressure of air. In ascending or descending elevations, the hand 

 or the Aneroid may be seen to move (like the hand of a watch), showing the 



