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THE POPULAR SCIENCE MONTHLY, 



come back to natural history, to anthropol- 

 ogy, 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 re- 

 member that it rests on a larger average in- 

 telligence, brought directly about by the dis- 

 covery 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 con- 

 stantly to modify existing ideas. But if we 

 can show the necessity for a constant modi- 

 fication of our ideas, arising out of our own 

 constitution, then it may be seen to be un- 

 reasonable to defame those who follow the 

 search for truth. And it being undoubted- 

 ly 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 ed- 

 ucation, we can see how wide-reaching the 

 effect of our improved basis of civilization 

 must be upon us as a people, and how im- 

 portant it is to understand the real direction 

 in which it works. 



The Causes of Ocean-Cnrrents. — An im- 

 portant contribution to the theory of ocean- 

 currents is made by Professor Zoppritz, of 

 the University of Giessen, who aims to show 

 that these currents are produced by the 

 impulsion of the winds. As Mr. Croll ob- 

 serves, in bringing Zoppritz'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 in- 

 fluence 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 unvary- 

 ing surface velocity makes itself felt with 

 linearly decreasing velocity down to the 

 bottom. Hitherto the view frequently ex- 

 pressed was, that the influence of surface 

 currents reached only to very moderate 

 depths. Secondly, it was found that all 

 variations according to time, whether pe- 

 riodic 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 sur- 

 face forces is determined by the mean ve- 

 locity of the surface, and that the periodic 

 variations are observable only Ln a compara- 

 tively thin surface stratum." 



Population-Density and Rates of Mortal- 

 ity. — 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 lY; at the other end, 31, 33, and 

 39. The acres per capita in the correspond- 

 ing districts are 12, 4, and 3, and I'Ol, "05, 

 I'Ol. The intermediate rates of mortality 

 are 18, 19, 20, 21, 22, 23, 24, and 25, while 

 the acres /)c;" capita are 4, 3'3, 2*9, 2'1, I'l, 

 •05, and "02. Now, excluding the London 

 districts, about which there is some diflBculty, 

 we have 7 groups of districts where the 

 mortality ranges thus : IV, 19, 22, 25, 28, 



