MARS AS A WORLD. 175 



accuracy, speak of the period as thirteen lunar months. 

 We have, therefore, two systems to compare. In one of 

 these the period of revolution is 8,000 months. In the 

 other, the period of revolution is thirteen months. In 

 each case the distance from the central body to the 

 revolving body is the same. The first is, of course, the case 

 of the earth and the fictitious moon, the second is the case 

 of the sun and the earth. The advantage of this method 

 of treatment is that we have by an artifice brought the 

 two distances into equality, so that they need not be 

 further considered. 



Look now at the two figures exhibited on the previous 

 page. They represent equal circles. In each case there 

 is a great attracting body in the centre, while the revolv- 

 ing body describes the circumference of the circle. The 

 circumstances, it will be seen, are quite similar, except 

 that there is a marked difference between the periods of 

 the two revolutions. To what can this difference be due P 

 As the distances are the same, the discrepancy in the 

 periodic times can be only attributed to the difference in 

 masses between the central bodies. We can show that the 

 mass or size of the revolving body has no appreciable influ- 

 ence in the matter. At a specified distance from its 

 primary, a satellite will revolve in substantially the same 

 time, whether its mass be only an ounce or a million of tons. 

 Start any object around our earth with the right velocity 

 and at the right distance, and it will perform the circuit 

 in 8,000 months. We have a quite analogous pheno- 

 menon on our earth. A heavy body and a light body will 

 fall to the ground in the same time. Take a bullet in one 

 hand and a cork in the other, drop them at the same 



