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such precision that the results of the two latest investigators do not differ by a thousandth part. These were Professor Boys, F.R.S., of Oxford, England, and Dr. Karl Braun, S.J., of Marienschein, in Bohemia. They worked independently at the problem, meeting and overcoming innumerable difficulties one after another, getting greater and greater delicacy and precision in their apparatus, and finally published their results almost at the same time, the one in England, the other in Austria. The outcome of their experiments is that the mean density of the earth is slightly more than five and a half times that of water. This is a little less than the density of iron, but much more than that of any ordinary stone. As the mean density of the materials which compose the earth's crust is scarcely more than one half of this amount, it follows that near the centre the matter composing the earth must be compressed to a density not only far exceeding that of iron, but probably that of lead.

The attraction of mountains has been known for more than a hundred years. It was first demonstrated by Maskelyne about 1775 in the case of Blount Schehallion, in Scotland. In all mountain regions where very accurate surveys are made the attraction of mountains upon the plumb line is very evident.

We know that the earth rotates on an axis passing through the centre and intersecting the earth's surface at either pole. If we imagine ourselves standing exactly