Page:The evolution of worlds - Lowell.djvu/272

232 to have had the wrong end of the last collision is alone left hopelessly behind.

Another factor also is concerned. The smaller the planet, the lower the utmost velocity it can control, and the quicker, therefore, it must lose its atmosphere. For a greater number of molecules must at every instant reach the releasing speed. Thus those bodies that are little shall, perforce, have less to cover themselves withal.

Now this inevitable depletion of their atmospheric envelopes, the aspects of the various planets strikingly attest. They do so in most exemplary fashion, according to law. The larger, the major planets, as we have already remarked, have a perfect plethora of atmosphere, more than we at least know what to do with in the way of cataloguing yet. The medium-sized, like our own Earth, have a very comfortable amount; Mars, an uncomfortable one, as we consider, and the smallest none at all. All the smaller bodies of our system are thus painfully deprived so far as we can discover. We are certain of it in the case of our Moon and Mercury, the only ones we can see well enough to be sure. In further evidence it has been shown at the Yerkes and at Flagstaff that no perceptible effect of air betrays itself in the spectroscopic imprint of the rings of Saturn, those tiny satellites of his, and very recently a spectrogram of Ganymede, Jupiter's third moon, made at Flagstaff for