THE MOOS.

53

mean density as approaching six times that of water, we meanthat a globe of the size of the Earth , if solid, and of uniformcomposition throughout, would have to be made of somesubstance nearly six times as heavy as water, in order to beof the same weight as the Earth . But we cannot be at allcertain that the earth is solid throughout, there being manyreasons in favour of a contrary supposition. We are, moreover,sure that it is not composed of materials of uniform density.

It is consequently more advisable, in an elementary dis-cussion such as the present, to speak of the weight of theMoon as a whole, independently of any consideration as to itssolidity or uniformity of constitution, rather than to attemptto compare its mean density with that of the earth or ofwater.

Let us therefore now proceed to notice some of the principalways in which the Moon may be weighed, all of which weshall find agree in proving that its weight is to that of theEarth in the proportion of about 1 to 81.

One method of investigation which has been successfullyused depends upon the fact, to which we have already brieflyalluded, that, if two bodies suspended in space, as are the Earth and the Moon , mutually attract one another, the smaller willnot simply revolve round the larger, but both will revolveround the common centre of gravity of the two. If one bemuch larger than the other, this centre of gravity will bemuch nearer to that one, and may even be within its surface.Indeed, in the actual case of the Earth and the Moon , thepoint in question is situated at a distance of only about 3,000miles from the Earth ’s centre, i.e. about 1,000 miles within itssurface.

Moreover, two such bodies will revolve as if attracted by amass placed at the above point equal to the sum of theirmasses. These statements depend upon an elementary butvery important theorem in dynamics. Keeping it in mind,we watch the Moon ’s movements. We neglect that part of itsmotion which it has in common with the Earth around theSun ; and, as accurately as we can, we allow for any com-paratively slight difference between the Sun’s attractions upon