The geometer who, more than two thousand years ago, set himself the task of measuring the circumference of the earth, is supposed to have made use of very much the same kind of implements as those employed by modern astronomers. He must have had a very fair instrument for measuring angles, and have known very well how to use it, seeing he was able to determine a value for the obliquity of the ecliptic which agrees so well with that established by modern science, its variations being, for what we know, taken into account; and for length or distance he would doubtless have some implement analogous to the metre, chain, foot-rule, or something called by other name that would, in those days, present facilities for selling a yard of calico. His operations would probably be as plain and simple as those applied to the measuring of a village green--for we are not told that he had any idea of there being any difference between the length of a degree of the meridian at the equator and one nearer either of the poles--and involved no hypotheses or theories, any more than modern operations have done. 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_:--