Flagstaff, 1896, and at Mexico in 1897, from which it appeared that the planet's markings were not obscured by cloud, but seen, as it were, through a veil, and which also showed the correctness of Schiaparelli's deduction that Venus in all probability turned in perpetuity the same face to the sun. That she did so was evident from the long-continued observations at Flagstaff and Mexico. Now such a facing always of one hemisphere sunward would cause convection currents upward in the centre of the disk, and an indraught along its edge, together with an absence of moisture on the sunlit half of the planet. Dry winds. of the sort blowing over a perpetual Sahara must be laden with dust, which Very's investigation finds to be the chief cause of reflection in our own air. The high albedo of Venus thus stands accounted for.

LIGHT AROUND VENUS.

A sidelight bearing on the albedo of air comes from the prolongation of the crescent of Venus when the planet passes in inferior conjunction before the sun.

It used to be thought that the fine circlet of light that then crowns the disk was due to refraction in the Venusian air. But in 1898 Russell, at Princeton, showed that it is rather reflection from that air than refraction through it which reaches our eyes. Now that such should be the case follows from the planet's albedo, if that albedo be of atmospheric and not of nubial origin. This supports the conclusion reached by the visual observations of Venus at Flagstaff. For refraction means transmission, and if the air of Venus reflects 90 per cent of the incident light, it can refract but 10 per cent at most. The light from it, therefore, must be reflected, not refracted, light in the proportion of nine to one. The albedo, Russell's observations, and the Flagstaff results, thus all concur to the conclusion that Venus is not enveloped in cloud.

DEDUCTION AS TO AMOUNT OF MARTIAN AIR.

Another outcome of the consideration of albedoes is a means it gives of approximating to the density of the Martian air. Mars is chiefly Saharan, and dust, therefore, must be largely present in its air. Now from the albedo of various rocks, of forests, and of other superficies, we may calculate the relative quotas in the whole albedo of Mars, of its surface and its air. Five eighths of its surface is desert, and therefore of an albedo of about .16, as its hue shows three eighths of a blue-green, the color of vegetation, with an albedo of about .7, while one sixth is more or less permanently of a glistening white in the

polar caps. These would combine to give it an albedo of .13. This, however, is illuminated by so much of the light as penetrates the atmosphere only, about three quarters of the whole. Whence the apparent albedo of the surface must be about.10. As the total albedo of the planet is .27, the remaining .17 is the albedo of its air.

Taking the density of the air as proportionate to its brilliancy, which would seem to be something like the fact, since the denser the air the more dust it would buoy up, we have for the Martian air a density about two ninths our own over each square unit of surface.

Now, if the original mass of air on each planet was as its own mass, we should have for the ratio between the Earth and Mars, 9.3 of atmosphere on the former to 1 on the latter. This being distributed as their surfaces, which are in the proportion of 79192 to 42203, must be divided by 3.5, giving 2.7 times as much air for the earth per unit of surface. The difference between 2.7 and 4.5 found above may perhaps be attributed to the loss of air Mars has since suffered on the supposition of proportionate masses to start with.

AIR DENSITY AT SURFACE OF MARS.

To get the relative density of the air at the surfaces of the two planets these amounts must be divided by the ratio of gravity at the surfaces of the two, that is, by .38.

For the density being proportional to its own increase, if D denote the density at any point, we have

where g denotes the force of gravity at the surface of the earth, and x is reckoned from that surface outward into space, whence

D= Ae-,

A being the density at the surface of the planet.

For Mars we have correspondingly

For the whole mass of air over a space dydz we have, for the Earth,

and as the whole mass of the earth's atmosphere over any space dydz 4.5 that of Mars at a similar point, and g1 = .38g, we have

whence as A = 30 inches of barometic pressure, A1 = 2.5 inches.

BOILING POINT ON MARS.

=

Owing to the less amount of the Martian air and the smaller gravity at the surface of the planet the boiling point of water is greatly reduced, being probably in the neighborhood of one hundred and eleven degrees Fahrenheit. If the whole mass of air be of the earth's, while gravity is .38 of ours, the pressure is

Mig1 = .09 of the earth's,

whence the boiling point is 44° C. or

79 +32=111° F.

1 4.5

For the same reason sublimation takes place more freely at identical temperatures there. Proportionally, therefore, there would be more water-vapor in the air.

We may summarize the results for Mars:

The look of the surface entirely corroborates the temperature result of this investigation.

JANUARY 14, 1907.

Proceedings of the American Academy of Arts and Sciences.

VOL. XLII. No. 26. —APRIL, 1907.

CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY, HARVARD UNIVERSITY.

THE TRANSMISSION OF RÖNTGEN RAYS THROUGH METALLIC SHEETS

BY JOHN MEAD ADAMS.

INVESTIGATIONS ON LIGHT AND HEAT MADE AND PUBLISHED, WHOLLY OR IN PART, WITH APPROPRIATION FROM THE RUMFORD FUND.