Page:Popular Science Monthly Volume 30.djvu/37

Rh and physical investigations have been initiated which give some promise of solving the mysterious problems of the sun's surface-drift, and the periodicity of the spots. We propose in this paper briefly to summarize these advances.

The transit of Venus on December 6, 1882, was widely, and, on the whole, successfully observed. The Americans alone used photography to any great extent, and at the nine different stations (four of them in the southern hemisphere) nearly fifteen hundred photographs were obtained, of which over a thousand are good for measurement. The German heliometer parties were also successful; and a great body of contact and micrometric observations and some photographs were obtained by French, English, and Belgian parties. The publication of the photographic and heliometric results is waited for with much interest, but, for some reason, has been greatly delayed. The general impression, however, is that the results will not prove as consistent and accurate as had been hoped, the probable error remaining still pretty large, and indicating that transits can not compete in accuracy with some of the other methods of determining the solar parallax.

Since 1882 the Washington experiments of Professor Newcomb upon the velocity of light have been completed and published, along with a new and independent determination by Michelson, at Cleveland. The anomalies in Newcomb's earlier observations were traced to their source and removed, and now the results of both observers stand in very close and gratifying accordance. Newcomb's is 299,860 kilometres, Michelson's 299,853.

To go with this in fixing the solar parallax, we have the new determination of the constant of aberration, by Nyrén, of Pulkova, based on all the Pulkova observations up to 1883. This value, 20"⋅492, combined with the above velocity of light, and with Clark's value for the earth's equatorial radius (6378⋅2 kilometres), gives for the solar parallax 8"⋅794—almost absolutely accordant with that deduced from the heliometer observations of Mars, in 1877. The observations of the eclipses of Jupiter's satellites, by Professor Pickering's photometric method, now in progress at Cambridge and Paris, will also give an extremely valuable result when the twelve-year cycle is completed. It has fixed the precise number of seconds required for light to traverse the mean distance between the earth and the sun.

The most remarkable result which has been arrived at, with reference to the solar radiation since 1882, is the fact, ascertained by Langley, that we do not receive from the sun any of the low-pitched, slowly pulsing rays, such as are emitted from surfaces at or below the temperature of boiling water. The solar spectrum appears to be cut off squarely at the lower end, and this cutting off we know can not have been effected in the earth's atmosphere, because we receive from the moon just the very kind of rays that are missing from the solar spectrum, and that in considerable quantity as compared with the rays of