sensible at the point held by the spring, and when the top descends the spring draws the piece of iron into close contact with it. This piece of iron acts as a lever, having its fulcrum at one extremity, the power at the centre of the box-top, and the other extremity controlled by the spring. Thus it is evident that the small motion of the centre of the box-top is much increased at the spring extremity. The motion thus obtained is communicated to a system of levers; and, by the intervention of a piece of watch-chain and a fine spring passing round the arbour, turns 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 bulges out under diminished pressure, the mechanical 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 that which a good mercurial barometer would at the same time and place indicate; that is, supposing it correctly adjusted. A different and more elegant arrangement has since been adopted. A broad curved spring is connected to the top of the vacuum box, so as to be compressed by the top of the box yielding inward to increased pressure, and to relax itself and the box as the pressure is lessened. The system of levers is connected to this spring, which augments and transfers the motion to the index, in the manner already described. Increase of pressure causes the levers to slacken the piece of watch-chain connected with them and the arbour of the index. The spring now uncoils, winds the chain upon the arbour, and turns the index to the right. Decrease of pressure winds the chain off the barrel, tightens the spiral spring, which thus turns the index to the left. The graduations of the aneroid scale are obtained by comparisons with the correct standard reading of a mercurial barometer, under the normal and reduced atmospheric pressure. Reduced pressure is obtained by placing both instruments under the receiver of an air pump. [Illustration: Fig. 33.]