Page:Popular Astronomy - Airy - 1881.djvu/262

248 to that at B. You must also remember what I have said about the pressure at E depending not only on the length of the pressing column of fluid, but also on the effective force upon each part of it. Then you will easily see that the effective force or gravity at B, must be to the gravity at A, in the proportion of 229 to 230. All this depends on the supposition that the fluid is of equal density thoughout.

The next point is, how can we verify this by observation? How can we find whether a body appears heavier upon one part of the earth than upon another? I have already said that it will not do to take a pair of scales and weigh the body with weights. The next suggestion is to weigh it with a spring-balance. It is not beyond possibility that a spring-balance might be made which would be sufficiently delicate for this purpose. The principal difficulty perhaps would be experienced in overcoming the effects of change of temperature in altering the elasticity of the spring: but if this could be done, and if the spring balance otherwise could be constructed with very great delicacy, the gravity at different parts of the earth could be compared. But the method which actually is used for this purpose, is one depending on the effect of gravity in producing motion. Theoretically, this effect of gravity may be measured by observing how far a stone falls in one second; but practically it is more accurately ascertained by the use of a pendulum. This is susceptible of very great accuracy indeed.

The pendulum is made of metal; it turns with a hard steel prism, having a very fine edge, upon hard plates of agate, or some very hard stone. It swings like the pendulum of a clock. But you must observe that a clock pendulum will not do for this purpose,