466 



NATURE 



[December 8, 192 1 



The chief experimental difficulty would be to originate 

 waves the length of which was only a small fraction 

 of the diameters of the rods. 



Since compression at the surface of a solid depends 

 on E, it would be just possible for a precipice to 

 exist where the height and density of the material 

 made the pressure at the base equal to the "crush- 

 ing " limit. Taking this limit from some of the text- 



■5A. 



Fig. I. — The ordinates of the curves show respectively the values of the 

 coefficients E and B as multiples of the rigidity (n) in terms of 

 Poisson's ratio (/i). 



books on the "strengths of materials," it appears that 

 the highest vertical face which could stand is, for 

 granite, marble, or gneiss, 8000-14,000 ft. ; various 

 limestones, 5000-9000 ft. ; various sandstones, 2500- 

 9000 ft. ; and various chalks, 500-2500 ft. 



I believe that no real precipice {i.e. vertical wall of 

 rock) is known which exceeds, even if it reaches, a 

 height of 6000 ft., nor is it to be expected that 



z, z 



Fig. 2. — OZ is the surface of the ground; 

 OV indicates a vertical plane wave 

 surface ; ZjYx indicates the same surface 

 after advancing in the direction OZ. 



weathering and other influences would allow of any- 

 thing like the maximum crushing stress to be borne 

 permanently at a free face. 



If a solid were compressed by a piston in a per- 

 fectly inextensible cylinder, the strain produced would 

 not be simple volume compression, but volume com- 

 pression combined with a certain amount of shear, 



NO. 2719, VOL. 108] 



and if the shearing strain exceeds the shear limit, 

 some molecular rearrangement must take place, 

 although the boundary conditions prevent any actual 

 rupture. The very large differences in the densities of 

 various chalks and limestones may, perhaps, be indi- 

 cations of the pressures to which they have been sub- 

 jected after their deposition. A. Mallock. 

 9 Baring Crescent, Exeter, November 11. 



The Action of Sunlight: A Case for Inquiry. 



Recently the Times published an article by Prof. 

 Benjamin Moore on the action of sunlight, and the 

 ensuing correspondence revealed an extreme contrariety 

 of opinion upon a subject of primary and ^profound 

 biological and civic importance. Up)on such a ques- 

 tion men of science should surely have reached some 

 agreement, but it is not so. 



Even amongst those who are certain of the super- 

 lative powers of sunlight against certain forms of 

 disease, which I have called the diseases of darkness, 

 there is extreme contradiction of opinion as to its 

 modus operandi, and this is not merely an academic 

 question, but vitally concerns our urban lives. For 

 about thirty years it has been believed that the healing 

 power of sunlight resides in its ultra-violet rays, which 

 are not light at all in the visible sense. If that be 

 true, it is so much the worse for us in London cm: 

 Glasgow or Liverpool or Belfast, who are near sea- 

 level, where the solar radiation is very poor indeed 

 in those rays. 



I have lately seen the work of Dr. Rollier at Leysin, 

 in the Alps, where ultra-violet rays abound. Since 

 then I have seen for myself the work, second only 

 to his, of the Treloar Hospital, under Sir Henry 

 Gauvain, at Hayling Island, on the English Channel. 

 The view of Prof. Benjamin Moore and Dr. Leonard 

 Hill that the ultra-violet rays are not useful, but 

 dangerous, needing exclusion by the development of 

 pigment in the skin, is thus supported. In a recent 

 letter to a contemporary, d propos an article of mine 

 on this subject. Prof. W. M. Bayliss has further ques- 

 tioned the value of the ultra-violet rays, and has 

 pointed to the need for an inquiry. 



It is clearly all-important for us to know whether 

 our ordinary English sun, "the light of common day," 

 provides us, at our customary humble levels of busi- 

 ness, with enough of the very kinds of radiation 

 which avail for life and against death. I believe 

 that it does, and my belief is' strongly confirmed by 

 the reading of the recent papers bv Dr. Carl Sonne, 

 of the Finsen Light Institute, at Copenhagen, accord- 

 ing to which the healing power of sunlight is due to 

 its familiar, visible rays. 



We expect at any moment the final report of Lord 

 Newton's Committee on Smoke Abatement. It will, ' 

 I am confident, be a cogent and valuable document. 

 Alreadv the time has come, predicted by _Dr. Hill a 

 few weeks ago, when we should be longing for the 

 sunlight — and the wise and well-to-do are following 

 the advice of railway advertisements to " Escape 

 Winter Fogs " by going to Monte Carlo. They are, 

 however, "statistically contemptible." The mass of 

 the population, upon whom our Empire chie^ ) 

 depends in peace and war, have to winter "very " 

 otherwise " — here, at least, but not, say, in Winnipeg 

 or Calgary, as I learnt there recentlv. 



Following the publication of Lord Newton's report, 

 let us have a co-ordinated inquiry into the action of 

 sunlight in health and disease, under the fortunate 

 auspices, to which alreadv we owe so much, of the 

 Medical Research Council and Sir Walter Fletcher, ; 



