When the time came for making efforts to ascertain the density of the earth, science seems to have employed the very simplest means it had at its disposal for attaining its object, and to have gone on refining its implements and operations in conformity with the lessons it went on learning while pursuing its self-imposed task. Every one who, even for recreation, has read a fair amount of the multitude of works and writings that have been published on Popular Astronomy--not to speak of text-books--knows that the first attempts were made by measuring the attraction of steep, or precipitous, mountains for plummets suspended in appropriate positions in their neighbourhood; then--evidently from knowledge acquired during these operations--by the attraction for each other of large and small leaden balls suspended on frames and torsion balances, which go under the name of the Cavendish Experiment; and afterwards by a refinement on this in using the Chemical Balance, where only one large and one small ball of metal are required. All these operations and their results are to be found described in works of various kinds, and are generally reduced to something like the following tubular form, which we reproduce in order to make more intelligible what we have just said, and that we may make a few remarks upon them.
There is no hypothesis, no theory, connected with any of the operations, unless it was the supposition that a plummet--which was naturally believed to point to the centre of the earth--should be pulled to one side by the attraction for it of a mountain in its neighbourhood, and that was found to be a fact.
METHODS EMPLOYED FOR FINDING THE DENSITY OF THE EARTH, AND THEIR RESULTS.
(1) _Deviation of Plummet by the Attraction of Mountains_:--
Experiments made. By whom, and Date. Mean Density found. At Schiehallien Maskelyne 1772 4·713 At Arthur's Seat Sir H. James 1855 5·316
Cavendish 1798 5·448 At Freyberg, Saxony Reich 1837 5·438 At Manchester Francis Baily 1838-1842 5·675
In the case of the plummet deviating from its absolutely straight direction towards the centre of the earth, caused by their attraction, not only the mountains themselves had to be measured and virtually weighed as far as they were measurable, but the weight of the wedge or pyramid between that measurable point, in each case, and the centre of the earth had to be estimated in some way; then the centre of gravity of the whole of this mass had to be ascertained, as well as the respective distances from the centre of the earth of this centre of gravity and that of the plummet, and only after all this and a deep study of the mutual attractions of this mass and the plummet could an estimate be formed of the mass of the earth. It will thus be seen that such measurements and estimates could never be looked upon as very exact and reliable; and nevertheless they have come very near the density of 5·66 finally adopted for the earth.