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able by means of it to measure with accuracy so small an at-traction as that of the large leaden balls,* which, in Mr. Baily’scase, were about one foot in diameter,—an attraction so smallin comparison with that of the Earth , that, in order for it toproduce a measurable displacement in the direction of a plumb-line, similar to that caused by the mountain in the Scliehallienexperiment, the plumb-line used would need to be of theimpracticable length of many miles. It is, moreover, a greatadvantage that the whole effect to be measured is producedhorizontally, so that the movements of the apparatus involveno alteration of the Earth ’s attraction upon any of its parts.
It might perchance be just possible to obtain a relationbetween the attraction of a large ball upon a small one andthe small one’s weight (which is equal to the attraction ofthe Earth upon it), by weighing the latter in a very delicatebalance, while the big one might alternately be placed ashort distance above it, and then below it, and noting thedifference caused in its apparent weight; but there wouldbe many more difficulties involved, and much less accuracywould be attainable, than in the Cavendish experiment, theslowness of the oscillations in which not only increases thedelicacy of the results deducible, but very much conduces totheir own accurate measurement.
Our readers will, we hope, understand, that, in any suchinvestigation the point to be arrived at is, to obtain a relationbetween the actual attractive effect of the Earth and that of agiven weight of some known substance, such as lead. Whenwe weigh two bodies against one another in any ordinarybalance, although the weight of each is produced by. theEarth ’s attraction, we only compare its attractions upon them ;we cannot in any case determine what that attraction abso-lutely is. But in the Cavendish experiment we obtain a directrelation between the actual attractive effect of the big balls,so far as it depends upon the known density of the material ofwhich they are made, and the effect which they would produce
* The balls were made of lead because of its weightiness, so that aslarge an attraction as possible might be obtained with balls of a moderatesize.