146 



NATURE 



[October 25, 1917 



internal energy, especially that associated with the 

 longer-period motions, will be in part energy of cur- 

 rents arising from temperature differences, and there- 

 fore supplied by the solar heat and not by the energy 

 of the earth's rotation. The contribution from the 

 shallower parts of the ocean may have more chance 

 of falling under the latter head. In any case, the 

 whole question can be discussed only when more is 

 known as to the distribution of the oceanic currents. 

 At present the only motion known at a considerable 

 distance from land is the residual drift, and this only 

 in a few places, chiefly where it exceeds one knot. 

 But as this does not change with the tide, its energy 

 is of thermal origin. 



It is more difficult to agree with Mr. Jeffreys 's con- 

 tention that viscous action in a solid earth cannot be 

 an appreciable cause of the slowing of its rotation. 

 By using a special law of viscosity quoted by him 

 (M.N. Roy. Astron. Soc, vol. Ixxvii., p. 449) as 

 suggested amongst other possibilities by Sir J. Larmor 

 as a reasonable alternative to the Maxwell-Darwin 

 law used by him previously (M.N. Roy. Astron. Soc, 

 vol. Ixxv., p. 648), he himself has considerably modified 

 his previous views. But a wide field of choice is 

 open, of which this is one example. Thus the law 

 might be that the ratio of the stress to strain is 

 n + f{d/dt), where / is any function. In order to give 

 the required values of the earth's retardation and of 

 the Eulerian nutation, the function / is defined for only 

 two values of the argument, and so is to a great extent 

 arbitrary. Evidently suitable forms may be chosen in 

 very many different ways, so as, in addition, to allow 

 for the properties of earthquake waves. 



R. O. Street. 



University of Liverpool. 



Stereo-Radioscopes. 



We have read with interest in the Notes columns of 

 Nature of October 18 a description of what is called 

 a stereo-radioscope, said to be invented by one Major 

 Lievre. What interests us so much is the fact that 

 Sir J. Mackenzie-Davidson invented the same thing 

 no fewer than twelve years ago. The instrument was 

 made by our firm and put on the market for several 

 years. As the two sources of rays have to be about 

 6 cm. apart, the only practical method was found to 

 be to build a special X-ray tube with two anti-cathodes 

 in the same bulb. 



The apparatus was exactly the thing described in 

 your paragraph. A motor drove an interrupter having 

 two contact blades opposite each other, exciting the 

 two sides of the tube alternately and driving a strobo- 

 scopic shutter synchronously with the interrupter. 



The great objection to the instrument is that the 

 operator must look into the view-box in front of the 

 shutter, thus fixing his position with regard to the 

 large and heavy instrument. Either this latter or the 

 patient must be adjusted to obtain the proper view. 



The difficulty of this is obvious, and results in an 

 expensive and cumbersome apparatus. 



Harry W. Cox and Co., Ltd. 



161 Great Portland Street, London, W.i. 



An Optical Phenomenon. 



Capt. C. J. P. Cave's letter in Nature of October 

 18 reminds me of a similar effect experienced when 

 travelling in a coupe compartment at the rear of a 

 train some years ago. From a window at the back 

 of the coup^ one could watch the ever-disappearing 

 landscape as the train travelled along. The impres- 

 sion created was that every object seen appeared to be 



NO. 2504, VOL. 100] 



rushing away from the train. But a stranger sensation 

 occurred when turning my eyes from the window to 

 objects in the coupe, for, during a space of a few 

 seconds, they appeared to be moving rapidly in a con- 

 trary direction. C. Carus-Wilson. 

 October 19. 



The effect described by Capt. Cave in Nature of 

 October 18 can be observed after walking rapidly along 

 the top of a wall and keeping the eyes fixed on the 

 road. On stopping, still looking at the road, part of 

 the field of view seems to be slipping away backwards.. 



H. M. Atkinson. 



45 Denman Drive, N.W., October 19. 



INFANT AND CHILD MORTALITY. 



POLITICAL economists are g-enerally agreed 

 that, if a country is to be prosperous and to 

 maintain its place among the nations, its popu- 

 lation must substantially and progressively 

 increase. Two cardinal factors are essential to 

 ensure a satisfactory increase of population : 

 (i) a birth-rate maintained at a proper level, and 

 (2) a death-rate not excessive. A falling birth- 

 rate and an excessive mortality are both national 

 calamities ; indeed, it may be questioned if France 

 would be quite in her present position had her birth- 

 rate equalled that of Germany. In France the birth- 

 rate, already abnormally low, fell from 23*5 per looa 

 in 1887 to 190 in 1914, while for Germany for the 

 same years the figures were respectively 369 and 

 283, with the result that during this period the 

 population of France only increased from about 

 38^ millions to 40 millions, whereas that of Ger- 

 many increased from 49 millions to 65 millions. 



We are in a .similar parlous state as regards 

 our birth-rate, for this has been steadily declining 

 from 363 in 1876 to 23*0 or thereabouts in 1916 

 per 1000 of population. The effect of this has 

 been that our increase of population for 1914 was 

 less by nearly half a million than it would have 

 been had the birth-rate obtaining in 1876 been 

 maintained. Fortunately, our mortality-rate is one 

 of the lowest in the world, and this, together with 

 a considerable saving of infant and child lives, 

 has enabled us to show a substantial increase of 

 population. We are, nevertheless, still faced with 

 a low and apparently falling birth-rate (for the 

 County of London the birth-rate was 21*5 for 1916, 

 against 22'5 for 191 5), and we must, moreover, 

 take into account the serious losses among the 

 adult male population, the potential and prospec- 

 tive fathers of children, owing to the present war. 

 It does not, therefore, appear that any substan- 

 tial Increase in the birth-rate can at present be 

 anticipated. 



We are, then, more than ever dependent upon a 

 diminution of mortality if our* increase of population 

 is to be maintained. But with a death-rate among 

 the whole civil population of 14' 7 per 1000 (191 6), 

 we can scarcely expect any considerable dimi- 

 nution in the general mortality. Is there any sec- 

 tion of the community among whom the mortality 

 is excessive and might be reduced? An analysis 



