458 



THE EARTH. 



the entire globe of the earth would exert in a body near it. with that which 

 a mass of matter of known weight, such as a mountain, would exert upon the 

 same body. The mode of executing that memorable experiment was as 

 follows : Let A B, fig. 9, represent a small portion of the earth's surface, 



which may be regarded as a plane ; lt:t C 1) represent a mountain, and let 

 be supposed to be its centre of gravity. The entire attraction of the mass of 

 the mountain will then be exerted as if it were concentrated on the point 0. 

 The direction of the earth's attraction will be perpendicular to the plane A B. 

 Now, let L be a weight suspended from any point ; M L forming what is called 

 a plumb-line. If the weight L were solicited by no force except the earth's 

 attraction, the string by which it is suspended would take a position at right 

 angles to the plane A B ; but as this plumb-line is suspended near the mount- 

 ain C D, it will be attracted by the gravitation of the mass of the mountain, 

 which will be exerted in the direction M O toward the centre of gravity of the 

 mountain. If we could imagine the globe of the earth on which the mountain 

 rests removed, and the mountain alone to remain near the plumb-line, then the 

 weight L would be drawn in the direction M 0, and the string M L suspend- 

 ing it would take that direction ; for in that case, the only force by which L 

 would be attracted would be the gravitation of the mountain, which takes place 

 in the direction M O. If, on the other hand, the mountain were removed, and 

 the earth alone left to affect the plumb-line, it would take the usual direction, 

 M L, perpendicular to A B ; but in the case actually supposed, the weight L is 

 solicited at the same time by both attractions by the attraction of the globe 

 of the earth drawing it perpendicularly to A B, and by the attraction of the 

 mountain drawing it in the direction M 0. By the common principles of me- 

 chanics, the weight L will in this case take a direction M L', intermediate 

 between M L and M O, leaning toward the mountain but very slightly, in- 

 asmuch as the attraction of the mountain is incomparably less than that of the 

 earth. 



Now, if we could exactly ascertain the degree in which the plumb-line is 

 deflected from its true vertical position by the attraction of the mountain, that 

 deviation or deflection will enable us immediately to estimate the proportion 

 which the attraction of the mountain bears to the whole attraction of the earth, 

 and that proportion would be the same as that which the weight of the moun- 

 tain or the mass of matter contained in it bears to the mass of matter contained 

 in the globe of the earth. But where the deviation of the plumb-line is so 

 small, and \vhre any ordinary test of its deviation would be affected by the 

 same cause as the plumb-line itself, there would be a difficulty in determin- 

 ing it. 



If the plumb-line were undisturbed by the mountain, its direction ought to 

 point to a star in the zenith of the place of the observer ; but being dis- 

 turbed by the attraction of the mountain, it will point to a star at one side of 

 the zenith say, for example, to the east of it. 



