Page:Popular Science Monthly Volume 86.djvu/219

Rh  21. Important studies of the spectra of spiral nebulæ and star clusters have been inaugurated. 22. An atlas of the moon was made in the first year of the observatory's existence, on the basis of photographs obtained with the large telescope. 23. The motions of approach and recession of about 1,500 naked-eye stars, distributed over the entire sky, have been observed with the 36-inch refractor at Mount Hamilton and the D. O. Mills reflector at Santiago.

 24. Spectroscopic observations at Mount Hamilton and on the summit of Mount Whitney have shown that the atmosphere of Mars is of low density, probably much less dense at the surface of Mars than the earth's atmosphere is at the summit of the highest peak in the Himalaya Mountains. These observations have established likewise that the quantity of water vapor in the atmosphere of Mars above, say, a square mile of its surface, must be very slight as compared with the quantity of water vapor in the earth's atmosphere above an equal area.