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THE POPULAR SCIENCE MONTHLY. 



bo prepared to find conditions under which 

 he would counsel irrigation, and other con- 

 ditions under which he would recommend 

 precipitation. Success at one place furnish- 

 es no argument that a process will be suc- 

 cessful everywhere. His own experience of 

 all kinds of schemes has led him to prefer 

 a scheme combining the principles of pre- 

 cipitation and irrigation. It has the advan- 

 tages that its efficient working is independ- 

 ent of the weather ; and that, if the works 

 are sufficiently large, any emergency of quan- 

 tity can be met. 



Atlantic and Pacific Fishes. Professor 

 David S. Jordan has published, in the " Pro- 

 ceedings of the United States National Mu- 

 seum," 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. Giinther, Dr. Steidachner, and Pro- 

 fessors Jordan and Gilbert ; and the work of 

 the few collectors who have given special at- 

 tention 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 sev- 

 enteen 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 faunae 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 op- 

 posed to the views expressed by Dr. Giin- 

 ther 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 compara- 

 tively recent date, a depression of the isth- 

 mus, producing an intermingling of the two 

 faunae. The discrepancy arises from the 

 comparatively limited representation of the 

 two faunae, at the disposal of Dr. Giinther. 

 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 as- 

 cend 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 arc substan- 

 tially distinct, so far as species are con- 

 cerned, 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 Observa- 

 tory," it appears that the first work in this 

 branch was an experiment made at the ob- 

 servatory in July, 1850, when, under the 

 direction of Professor W. C. Bond, a satis- 

 factory image of the star a Lyres was ob- 

 tained by Mr. J. A. Whipple. Subsequent- 

 ly, the double star a 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 experi- 

 ment was repeated with various stars and 

 clusters, but the work was finally aban- 

 doned, 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 re- 

 search 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 porl ions of the 

 sky, and of determining the light and color 

 of stars in all parts of the heavens. Pho- 

 tographs of the trails of polar stars no bright- 

 er than the eleventh magnitude were ob- 

 tained without clock-work. Stellar spectra 

 were obtained of the brighter stars without 

 clock-work, in which all the principal linos 



