Page:Popular Science Monthly Volume 6.djvu/773

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dam which will be exclusively occupied with sun-observations.

Carrington's work covered the period 1853-1861, and Spoerer's extended from 1861 to 1871, and was done on much the same plan. He, of course, confirms the discovery by Carrington of the law which declares that the velocity of the rotation of the outer layer of the sun's surface is greatest near the sun's equator and diminishes gradually toward the poles, and he arrives at other conclusions, a few of which we will give, as generally interesting, referring students of astronomy to the original work. It is well known, from the recent observations (since 1853), that Schwabe's discovery is true that the number of spots on the sun's disk is governed by some periodic cause which produces a maximum number and a minimum number of spots every eleven years. It is probable that magnetism, rainfall, and temperature, and other terrestrial phenomena, are connected with this period of eleven years, the cause of which is as yet unknown.

Spoerer has discussed the observations of the spots separately for each half (i. e., northern and southern) of the solar disk. He finds (page 137) that the points of maximum and minimum frequency of spots are reached earlier in the southern hemisphere of the sun. "While the minimum of the year 1856 still lasted in the northern hemisphere, the increase (in the number of spots) had already begun in the southern hemisphere, and had here in 1858 reached a maximum," while in the northern hemisphere the maximum began in 1860. The mean heliographic latitude of the spots, however, shows "no characteristic difference between the two hemispheres."

In the eleven-year period (1854-1864) the ratio of the number of spots in the northern hemisphere to the number in the southern was 933:1,000; from 1861-1871 this ratio was 976:1,000. The spots on the sun give a means of determining the velocity of the sun's rotation, but the determination of this element is complicated by the fact that these spots have a drift or proper motion in longitude. Spoerer finds the angle through which the sun rotates in one day to be 14° 16' nearly, while Carrington determined this element to be 14° 11'; that is, Spoerer fixes the time of the sun's rotation on its axis as 25 days, 5 hours, 37 seconds. This mean value—14° 16'—is, however, subject to change, and Spoerer suggests that further investigations may show that the changes which are known to occur in this value may be found to occur earlier in that hemisphere of the sun which has, at the time, the greatest spotted area. This question it will require several eleven-year periods to settle.

It is to be hoped that these most valuable researches will be continued, and that America will contribute her full share to the labor. There is a sure reward awaiting investigators in this field.

The Elevations of Certain Datum-Points on the Great Lakes and Rivers, and in the Rocky Mountains. by James T. Gardner, Geographer (from the Report of the United States Geological and Geographical Survey of the Territories for 1873). Washington: Government Printing-Office, 1875.

In this modest pamphlet of thirty pages Mr. Gardner has made a very valuable step toward utilizing a vast quantity of material which has until now been little employed. Every railroad or canal has been located only after one or two lines of leveling have been run between its terminal points, and a mere glance at a railroad map of the United States will show what an immense collection of data exists for the determination of the altitude of any point on any railroad.

A complete discussion of this has not been attempted, Mr. Gardner's principal object having been to determine the altitude of Denver, in Colorado, above the mean level of the Atlantic Ocean, Denver being the point to which the altitudes determined by the survey of the Territories are referred.

Incidental to this object, results of great interest have been obtained, a few of which will be mentioned. The material for the work was necessarily of the most varied character and of many degrees of accuracy, from the first trial-lines of reconnaissance-surveys to the final releveling of a finished railroad or canal. Great care was necessary in selecting from the reports of chief-engineers and elsewhere the right figures, and in giving proper weights to these when