# Page:Popular Science Monthly Volume 6.djvu/763

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CORRESPONDENCE.

CORRESPONDENCE.

RETARDATION OF THE EARTH'S ROTATION.

To the Editor of the Popular Science Monthly:

SIR: In the work entitled "Correlation and Conservation of Forces," edited by you, is an article on "Celestial Dynamics," by Dr. J. B. Mayer, in which is considered the subject of the diminution of the velocity of the rotation of the earth, in consequence of the retarding influence of the tidal wave. In the course of this article it is stated (page 297) that "theory and experience agree in the result that the influence of the moon on the rotating earth causes a motion of transition from east to west in both atmosphere and ocean," and that "this motion must continually diminish the rotatory effect of the earth, for want of an opposite and compensating influence."

The result of the retarding force of the waters on the axial motion of the earth is stated, by Dr. Mayer, to be the lengthening of the day 116 of a second in 2,500 years; and, further, that this retarding force will eventually diminish and exhaust the earth's rotatory motion.

The theory of Laplace, that opposite aerial currents are, as to their effects upon the rotation of the globe, as ${\displaystyle =}$ 0, and generally, that there are no forces at work in Nature which do not in Nature find their compensating opposites, is held by Dr. Mayer not to be applicable to the effect of this tidal translation of waters. Nevertheless, he proceeds to demonstrate, in the article on "The Earth's Interior Heat," that the decrease of temperature has been far too great, in the last 2,500 years, to have been without sensible effect in accelerating the velocity of the earth's axial motion. It is there shown that the loss of heat and consequent contraction of the earth's surface, in the time above mentioned, must have had the effect of accelerating the earth's rotatory motion to the extent of counteracting the retarding movement of the tides.

Now, if the results of these forces are equal one to the other, the day remains constant, and the law of Laplace would seem to apply as perfectly in this case as in that of opposing aërial currents.

 T. H. S.⁠ ⁠Cheyenne, W. T., February 16, 1875.

THE NEST AND EGGS OF THE THISTLE-BIRD.

To the Editor of the Popular Science Monthly:

The yellow-bird (Chrysomitris tristis) is exceedingly abundant throughout New Jersey during eight months of the year, and not at all rare during the winter, when it is seen mostly in loose flocks. For several years I have been perfectly familiar with its manner of nesting, and the appearance of the eggs, etc., and I can give my testimony to the correctness of the statements of Wilson, who says these birds cover the nest "on the outside with pieces of lichen, which they find on the trees and fences." In looking over the first volume of the American Naturalist, p. 115, I noticed a statement, made by Dr. T. M. Brewer, that he "never saw one that was ever covered on the outside with lichen;" and, in "North American Birds," vol. i., p. 474, he describes a nest from Wisconsin as typical, which, on the exterior, was solely "fine vegetable fibres." Here, in New Jersey, and it was from New Jersey nests (or Pennsylvania) that Wilson took his description, the nests are invariably lichen-covered. I believe I am justified in using so positive a term. I know that here, also, two broods are raised. So much for the nests.

I have seen many sets of eggs, probably 200, within ten years, and I know that splotches of yellowish-brown, and occasionally purple, are common on the larger end of the egg; not the rule, perhaps, but common, i. e., perhaps thirty per cent, of the eggs laid.

 Charles C. Abbott, M.D.⁠ ⁠Trenton, N. J., February 16, 1875.