polar cap, it suffices here to chronicle the fact that it took place; for the fact of its occurrence constitutes proof positive of the presence of an atmosphere.
A moment's consideration will show how absolutely positive this proof is. It is the inevitable deduction from the simplest of observed facts. Its cogency gains from its very simplicity. For it is independent of difficult detail or of doubtful interpretation. It is not concerned with what may be the constitution of the polar caps, nor with the character of the transformation that sweeps, wave-like, over the rest of the planet's face. It merely takes note that change occurs, and that note is final.
Now, since this was originally written, certain observations made at this observatory by Mr. Douglass have resulted apparently, most unexpectedly, in actually revealing this atmosphere to sight. Although the existence of an atmosphere is absolutely established by the above considerations, it is interesting to have ocular demonstration of it to boot; and this the more, that it would not have been thought possible to detect what, so to speak, disclosed itself. For the discovery was quite unconsciously made, being of the nature of a by-product to the outcome of another investigation. So systematically was his general search conducted that when the results came to be worked out it