Zo8 OF THE DISTANCES AND MAGNITUDES

sider this, and you will find it a subject worthyof your admiration and wonder.

The distance of the sun from the earth beingthus found, the distances of all the rest of theplanets may be easily determined, by the statedlaws of nature. For it was discovered by Kep-ler, from observation, that the squares of theperiodic times, in which the planets performtheir annual revolutions, are in proportion toeach other as the cubes of their mean distancesfrom the fun ; so that the distance of any oneof them being known, the distance of any othermay be easily determined. Suppose, for exam-ple, that I wanted to know the distance of Sa-turn from the sun; this may be performed bythe rule of proportion, as follows: As the squareof the time in which the earth performs herrevolution round the fun, is to the square of thetime in which Saturn performs his revolutionround the fun, so is the cube of the earth’smean distance from the fun, to the cube of themean distance of Saturn ; and if the cube rootof this last number be taken, it will give thedistance of Saturn from the fun, as was required.

And in a manner, equally easy, may the realdiameters and bulks of the planets be deter-mined, from their apparent diameters and dis-tances being known. The fun and moon, forinstance, appear nearly of the fame magnitude;and therefore, if the fun's distance from the

earth