Page:Popular Science Monthly Volume 14.djvu/704

686 come back to natural history, to anthropology, at last. A large class of persons with a certain bias persistently decry our modern civilization, and look for its more or less speedy evanishment, merely because Rome perished and Greece decayed. But nowhere in nature is there exact repetition, and to understand the new civilization we must remember that it rests on a larger average intelligence, brought directly about by the discovery of the art of printing. There is then a distinct reason, a scientific ground, for the opinion that our present civilization rests upon a surer basis than did those which preceded it, and this we may safely bring forward in the cause of truth. For science is in danger always of being regarded as the enemy of the state, because it tends constantly to modify existing ideas. But if we can show the necessity for a constant modification of our ideas, arising out of our own constitution, then it may be seen to be unreasonable to defame those who follow the search for truth. And it being undoubtedly true, as Locke says, that, of all the men we meet with, nine out of ten are what they are, good or evil, useful or not, by their education, we can see how wide-reaching the effect of our improved basis of civilization must be upon us as a people, and how important it is to understand the real direction in which it works.

The Causes of Ocean-Currents.—An important contribution to the theory of ocean currents is made by Professor Zöppritz, of the University of Giessen, who aims to show that these currents are produced by the impulsion of the winds. As Mr. Croll observes, in bringing Zöppritz's paper under the notice of English readers in "Nature," one of the main objections urged against this theory is that the winds can produce only a surface movement, while many of the ocean-currents extend to great depths. The reply to this objection is, that if the surface waters be impelled forward with a constant velocity by the wind or any other cause, they will drag along with themselves, though with a velocity somewhat less, the layer immediately below themselves. This second stratum now exerts the same influence on a third adjoining stratum, and sets it in motion in the same direction. The third stratum, in like manner, draws with itself a fourth, and so on. The propagation of this velocity is only bounded by the limits of the fluid itself. If these limits consist of a solid plane parallel to the strata, then the propagation of the velocity will cease only at that point, i. e., between the last liquid stratum and the first solid stratum. Among the results found, the author lays particular emphasis on two: "In the first place, the steady motion arising in the interior of an unlimited stratum of water from an unvarying surface velocity makes itself felt with linearly decreasing velocity down to the bottom. Hitherto the view frequently expressed was, that the influence of surface currents reached only to very moderate depths. Secondly, it was found that all variations according to time, whether periodic or aperiodic, of the forces acting on the surface, propagate themselves downward with extraordinary slowness, the periodic in very quickly decreasing amount. Taking both statements together, it follows that the movement of the chief part of the stratum of water exposed to periodically varying surface forces is determined by the mean velocity of the surface, and that the periodic variations are observable only in a comparatively thin surface stratum."

Population-Density and Rates of Mortality.—Some curious and interesting results are developed by Dr. Farr, F. R. S., from a study of the rate of mortality in connection with statistics of population-density. He finds that the rate of mortality increases as density of population increases, and this he proves by arranging the 619 districts of England and Wales in groups according to the rates of mortality, and showing that all the groups follow this law. Thus in the ten years 1861-’70, at one end of the scale the deaths per 1,000 of population are 15, 16, and 17; at the other end, 31, 33, and 39. The acres per capita in the corresponding districts are 12, 4, and 3, and 1·01, ·05, 1·01. The intermediate rates of mortality are 18, 19, 20, 21, 22, 23, 24, and 25, while the acres per capita are 4, 3·3, 2·9, 2·1, 1·1, ·05, and ·02. Now, excluding the London districts, about which there is some difficulty, we have 7 groups of districts where the mortality ranges thus: 17, 19, 22, 25, 28,