Page:Popular Science Monthly Volume 31.djvu/588

572 be prepared to find conditions under which he would counsel irrigation, and other conditions under which he would recommend precipitation. Success at one place furnishes no argument that a process will be successful everywhere. His own experience of all kinds of schemes has led him to prefer a scheme combining the principles of precipitation and irrigation. It has the advantages that its efficient working is independent of the weather; and that, if the works are sufficiently large, any emergency of quantity can be met.

Atlantic and Pacific Fishes.—Professor David S. Jordan has published, in the "Proceedings of the United States National Museum," a list of the fishes known off the Pacific coast of America, from the Tropic of Cancer to Panama. Our knowledge of these species is due chiefly to the studies of Dr. Gill, Dr. Günther, Dr. Steidachner, and Professors Jordan and Gilbert; and the work of the few collectors who have given special attention to the subject has in nearly all cases been of exceptional value. Of the four hundred and seven species of fishes now known from the Pacific coast of Mexico and Central America, seventy-one species, or seventeen and a half per cent, are also found on the Atlantic coast. If we add to this some eight hundred species, now known, from the Caribbean Sea and adjacent shores, we have about six per cent of the whole number known as common to the two coasts. With only this proportion of common species, the two faunæ show no greater resemblances than the similarity of physical conditions on the two sides of the continent would lead us to expect. This conclusion is opposed to the views expressed by Dr. Günther in his "Fishes of Central America," where he assumes that nearly one third of the total number of species of marine fishes on the two shores of tropical America will be found to be identical. Hence he infers that there must have been, at a comparatively recent date, a depression of the isthmus, producing an intermingling of the two faunæ. The discrepancy arises from the comparatively limited representation of the two faunæ, at the disposal of Dr. Günther. Several of the identical species are pelagic fishes common to most warm seas. Others are almost cosmopolitan in the tropical waters; while most of the rest often ascend the rivers of the tropics. We may account for their diffusion, perhaps, as we account for the dispersion of fresh-water fishes on the isthmus, on the supposition that they may have crossed from marsh to marsh at some time in the rainy season. Professor Jordan is therefore brought to the conclusion that the fish fauna of the two shores of Central America are substantially distinct, so far as species are concerned, and that the resemblance between them is not so great as to necessitate the hypothesis of the recent existence of a channel across the isthmus.

Progress of Stellar Photography.—From a paper by Professor E. C. Pickering, on "An Investigation in Stellar Photography, conducted at the Harvard College Observatory," it appears that the first work in this branch was an experiment made at the observatory in July, 1850, when, under the direction of Professor W. C. Bond, a satisfactory image of the star α Lyræ was obtained by Mr. J. A. Whipple. Subsequently, the double star α Geminorum gave an elongated image, evidently due to its two components. Objects as bright as these gave but faint images, and no impression was obtained from the pole-star, however long the exposure continued. The experiment was repeated with various stars and clusters, but the work was finally abandoned, owing to the imperfections of the driving-clock and the lack of sensitiveness of the plates. Both of these difficulties were partially remedied in 1857; the research was resumed by Professor G. P. Bond, and the value of stellar photography as a means of determining the positions and brightness of the components of double stars was established. The present research was undertaken in 1882, when it was shown that photography could be used as a means of forming charts of large portions of the sky, and of determining the light and color of stars in all parts of the heavens. Photographs of the trails of polar stars no brighter than the eleventh magnitude were obtained without clock-work. Stellar spectra were obtained of the brighter stars without clock-work, in which all the principal lines