not far therefore from the value found above, 1/190.

The second bit of corroborative testimony comes from the behavior of the satellites of the planet. Unlike a sphere, a spheroid acts unequally upon a body revolving about it in an ellipse inclined to its equator. The ring pulls the satellite now this way, now that, thus altering its nodes, that is, the points where the plane of its orbit crosses the planet’s equator, and also its apsides, or the points in which the satellite’s orbit is nearest and farthest from the planet. The effect of an equatorial protuberance tilted thus is to shift these points round the orbit, the line of nodes retrograding, while contrarily the line of apsides advances. From the speed with which these revolutions take place, it is possible to calculate the size of the bulge. Hermaun Struve has just done this for the lines of apsides of the two satellites of Mars, and finds for the value for the consequent polar flattening of the planet 1/190 of its equatorial diameter. From these two independent determinations we may conclude that the value found at Flagstaff is pretty nearly correct.

We find, then, that Mars is a little flatter than our Earth, though not noticeably so, the polar flattening amounting to about 22 miles.

The value, 1/190, for his polar flattening, hints that at some past time Mars was in a fluid—