January 23, 1896] 



NA TURE 



269 



myself alxDut the year 1854, and in which I found a velocity 

 comparable with the velocity of light. We did not then know 

 the relation between electro-static and electro-magnetic units. 

 If we work that out for the case of air instead of gutta-percha 

 we get simply v (that is, the number of electro-static units in 

 the electro-magnetic unit of quantity) for the velocity of propa- 

 gation of the impulse. That is a very different case from this 

 very rapidly varying electrification I have ideally put before you, 

 and I have waited in vain to see how we can get any justification 

 oi the way of putting the idea of electric and magnetic waves in 

 the so-called electro-magnetic theory of light. 



'• I may refer to a little article of mine in which I gave a sort 

 of mechanical representation of electric, magnetic, and galvanic 

 forces — galvanic force I called it then, a very badly-chosen 

 name. It is published in the first volume of the reprint of my 

 l)apers. It is shown in that paper that the static displacement 

 of an elastic solid follows exactly the laws of the electro-static 

 force, and that rotatory displacement of the medium follows 

 exactly the laws of magnetic force. It seems to me that an 

 incorporation of the theory of the propagation of electric and 

 magnetic disturbances with the wave theory- of light is mo.et 

 l)robably to be arrived at by trying to see clearly the view that I 

 am now indicating. In the wave theory of light, however, we 

 shall simply suppose the resistance to compression of the lumi- 

 niferous ether, and the velocity of propagation of the condensa- 

 tional wave in it, to be infinite. We shall sometimes use the 

 words ' practically infinite ' to guard against supposing these 

 (juantities to be absolutely infinite." 



The second extract which I give is from p. 143 of the Papy- 

 rograph edition of the "Baltimore Lectures" — a portion not 

 yet reprinted. 



" The want of indication of any such actions is sufficient to 

 l^rove that if there are any in nature, they must be exceedingly 

 small. But that there are such waves, I believe, and I 

 believe that the velocity of propagation of electro-static force is 

 the unknown condensational velocity that we are speaking of. 



" I say ' believe ' here in a somewhat modified manner. I do 

 not mean that I believe this as a matter of religious faith, but 

 rather as a matter of strong scientific probability." 



J. T. Bottom LEY. 



13 University Gardens, Glasgow, January 16. 



The Astronomical Theory of the Ice Age. 



May I first acknowledge the gentle kindliness with which my 

 early teacher and friend, Sir Robert Ball, has pointed out my 

 error in quoting from the old edition of his work. I much 

 regret that I did not make further inquiries, but I was satisfied 

 when the library clerks at Trinity College, Dublin, told me that 

 if there had been any alteration in the text, they would have 

 received a copy of the second edition. It appeared from Sir 11. 

 Howorth's letter that the mistake originated with the publishers, 

 who erroneously informed the library agent that the second 

 edition was a mere reprint, and therefore refused to supply a 

 copy. 



Both Sir Robert Ball and Dr. Wallace, in their letters in Nat u RE 

 of January 9, have misunderstood the way in which I present 

 my argument. If Dr. Wallace would read my papers again, I 

 think he will see that, so far as I am concerned, the whole of his 

 letter is founded on a complete misapprehension ; and Sir Robert 

 Ball will, I hope, also agree that he has somewhat altered the 

 form in which I have stated my conclusions, and that I have fully 

 recognised the difference which he thinks I have ignored. But 

 as the matter really at issue is the present position of the astro- 

 nomical theory, I may be excused from discussing this misunder- 

 standing further, for even if every word of their criticisms on my 

 conclusions were valid, the astronomical theory, as it issued from 

 the labours of Croll and Ball, would be in no better position than 

 before. Whether I am right or wrong in my belief that the 

 astronomical factor cannot have been the principal one, I venture 

 to think there can be no doubt that the existing exposition of 

 that theory must be given up. 



The foundation of the astronomical theory is the fall in tem- 

 perature directly due to diminished sunheat. Croll and Ball 

 accordingly give calculations which indicate a large fall. Croll 

 gets 45'y F. for the lowering of mid-winter temperature in tlreat 

 Britain during the long excentric winter, and Ball's modification 

 of Croll's method gives about 25° V. as the lowering of the 

 winter temperature. The first five pages of my article in the 

 Phi/. Mag. for December 1894 are devoted to showing that 



NO. 1369, VOL. 53] 



there is no justification for the principle on which this calculation 

 is made, and that the fall must be a mere fraction of that pos- 

 tulated in either exposition of the astronomical theory. The 

 chief flaw in the calculation is, curiously enough, that which Sir 

 Robert and Dr. Wallace erroneously attribute to me, viz. that 

 of considering that changes in terrestrial temperature are 

 directly proportional to the changes in sun-heat, and ignoring, 

 the important element of storage and transference by ocean and 

 air currents. How unsafe this is may be judged from the fact 

 that if the method used to calculate the temperature in the Glacial 

 Age from that in the present day were applied to find the 

 summer temperature from the winter temperature, we should 

 find for the British Isles a summer temperature of above 300° 

 F. if we take Ball's hypothesis, and some thousands of degrees 

 Fahrenheit if we take Croll's. If we calculate the winter tem- 

 perature from the summer one, we should get - 125° ¥. for our 

 winter temperature. A method which gives results in such 

 striking contrast to the truth can hardly be accepted as a basis 

 for a scientific theory. 



If, therefore, this first portion of my criticism be correct (and 

 hitherto no attempt has been made to refute it) the astronomical 

 hypothesis is in just the position it would occupy if neither 

 Croll's nor Ball's book had been written. So far, the hypothesis: 

 itself may be true or false ; it is only the reasoning which has 

 been put forward in its support that has to be abandoned or 

 modified. The theory is, as all will admit, a tempting one, and 

 accordingly I sought for some other means of establishing it. 

 After several fruitless efforts to hit on a fairly satisfactory method 

 of estimating the direct effect of an altered distribution of sun- 

 heat on terrestrial temperatures, the method which Prof. Darwirk 

 has described occurred to me, and from it, combined with a 

 discussion on the transjerence of heat by the Gulf Stream (see 

 Phil. Mag. December 1894, p, 548 and p. 551), I was led to 

 infer that for the British Isles at least the glaciation could not 

 with any degree of probability be attributed to the long winter 

 of great excentricity. 



Sir Robert Ball's views, as presented in his letter, seems to- 

 involve a return to Croll's point of view, at least to the extent 

 that the purely astronomical reason requires to be supple- 

 mented by a discussion of the oceanic and atmospheric currents. 

 This view appears to me a true one ; the only hope for the 

 astronomical theory would be to show that the adjustment of 

 terrestrial temperatures by the interaction of ocean and air cur- 

 rents with direct sun-heat is such that a very slight alteration of 

 sun-heat produces a very great alteration of temperature ; so that 

 if the sun-heat which falls on Cornwall in winter were to be 

 reduced to that which falls on Yorkshire, with corresponding 

 changes for the temperate latitudes, and somewhat greater ones 

 for the tropical belt, the ultimate result would be an Ice Age. 

 But how can we hope to establish such a theory when we 

 remember what a comparatively small change of temperature is 

 due to the far greater changes of sun-heat from equator to pole 

 as summer gives way to winter? Edwd. P. Culverweli,. 



Trinity College, Dublin, January 14. 



Changes of Length in Bars and Wires of Magnetic 

 Material due to Magnetisation. 



The appearance of a paper, by Dr. L. T. More, on the 

 changes in length produced in iron wires by magnetisation (Phil. 

 Mag., October 1895, p. 345, and Physical Review, vol. iii. p. 

 210), has drawn my attention to a curious divergence of opinion 

 on a fundamental point in magnetism. Dr. More has attempted 

 to analyse the change of length accompanying magnetisation 

 into a " direct action"' which " may possibly be due to the orient- 

 ing of the magnetised particles," and to ' ' indirect actions." He 

 adds (p. 224): " These indirect actions are the mechanical stresses 

 created in the rod by the magnetism. The first of these . . . 

 is the tractive force of the magnet and is measured by B^St. 

 That this force exists, tending always to contract the rod (italics 

 mine), is seen from the fact that if the magnet is cut in two, the 

 ends are held together. . . . This effect for high intensities of 

 magnetisation is a large one, and becomes one of the most im- 

 portant factors in the observed changes in length." The stress 

 referred to by Dr. More is that usually associated with the name 

 of Maxwell (" Electricity and Magnetism," vol. ii. Arts. 641 et 

 sc(/. Cf Ewing, " Magnetic Induction in Iron and other 

 Metals," § 147)- 



The first to propound the view adopted by Dr. More — that 

 the mechanical force tends to shorten the rod— would seem to be 



