May 30, 1 901] 



NA TURE 



107 



doubtedly be more severe than at present, while the 

 northern winter would be to some extent mitigated. Pre- 

 cession would have the effect of causing this state of 

 things to alternate in the two hemispheres. The argument 

 is perfectly simple and definite, and can only be ques- 

 tioned on the score of degree, not of fact. It thus passes 

 from the domain of astronomy to that of meteorology. 

 Here the ground is very debatable, and Mr. Culverwell's 

 discussion of the solar radiation incident on individual 

 zones of the earth may be regarded as a decided contri- 

 bution to a study of this part of the question. But his 

 researches do not seem to be necessarily so conclusive 

 against the astronomical theory of an Ice Age as this 

 author supposes. For the present purpose, however, 

 enough has been said as to the application of the theory 

 to our own planet. 



The conditions which hold in the case of Mars resemble 

 in a remarkable manner those which hold in the case of 

 the earth, except in one particular. The eccentricity is 

 •0933, and hence b = "i i. The constant a = '27. These 

 values of a and b apparently afford an excellent example 

 of the conditions required by the theoiy for a glacial 

 epoch in the southern hemisphere of Mars. It would be 

 illogical to compare the state of corresponding regions 

 on the earth and on Mars, because the concomitant 

 circumstances cannot be expected to be the same. But 

 it is reasonable to suppose that in corresponding seasons 

 the latter planet should show different phenomena in the 

 two hemispheres. As an index of these variations we 

 naturally look to the polar caps, the size of which is known 

 to vary greatly with the seasons. The results to be expected 

 are of this kind : 



The amount of heat during summer in either case being 

 the same, there is room for question as to the relative 

 size of the two minima. But it seems clear that the 

 radiation incident on the southern hemisphere will be 

 transformed into heat at a higher temperature than in 

 the northern. According to the laws of thermodynamics 

 it should, therefore, be more effective in the south. The 

 total energy received is the same, but the available energy 

 is greater. Or the matter may be considered differently. 

 It is quite conceivable that during the long cool summer 

 the temperature would not rise above the melting point 

 of ice or whatever substance is in question, while it would 

 be otherwise during the short hot summer. An approach 

 to this condition would lead us to expect the more pro- 

 nounced minimum in the southern hemisphere. 



To compare the suggestions of theory with the facts of 

 observation, we can turn to a recent paper on the subject 

 by Mr. Lowell.' He concludes that the caps are com- 

 posed of ice and not carbonic acid, on account of the 

 difficulty of obtaining the latter substance in a liquid 

 form, a state which he considers proved by observation 

 to exist on Mars. He expresses a belief that the Martian 

 sky is cloudless during the day and that the surface of 

 the planet is protected by cloud at night. It must be 

 confessed that the regularity of transition at sunrise and 

 sunset from one condition to the other almost surpasses 

 our belief and tends to discredit the deductions from the 

 study of projections upon the terminator. Mr. Lowell 

 then considers the evidence of the polar caps. The 

 result is to confirm the expectations formed above, and 

 the conclusion is perhaps the more convincing because 

 the writer seems to find the phenomena surprising. And 

 this point is rather important, for the evidence as to the 

 maxima is not so complete as could be desired. It is to 



1 " Mars on Glacial Epochs," by Percival Lowell : Proceedings of the 

 American Philosophical Society, Nov. i6, igoo. 



NO. 1648, VOL. 64] 



be hoped that Mr. Lowell's examination of this part of 

 the question will lead to the acquisition of more complete 

 and satisfactory evidence. 



With regard to the minima, Mr. Lowell does not give 

 the simple explanation suggested above, but attributes 

 the phenomena observed to a deficiency of precipitation. 

 In fact, he professes to prove that "as the precipitation 

 increases a time must come when the southern minimun> 

 will actually exceed the northern one in size, aiid do sa 

 more and more, indcfiiiitiiy." As no physical principle 

 is invoked to account for this result, it would seem to be 

 based on assumption. Examination shows that the 

 assutnptions tacitly involved are (i) that the maximum of 

 each cap is (increased in the same ratio, (2) that the 

 amount melted at each cap remains exactly the same as 

 before. It would then follow that in each hemisphere 

 the rate of proportionate growth of the minimum would 

 exceed that of the maximum. We now introduce the 

 observed fact that the ratio of maximum to minimum is 

 greater in the southern hemisphere. This, added to the 

 foregoing, is necessary and sufficient to prove that the 

 rate of proportionate growth of the minimum is greater 

 in the southern than in the northern hemisphere. But 

 the observed fact employed might equally well have 

 been stated in this form : comparing the southern with 

 the northern hemisphere, the ratio of the minima does 

 not exceed the ratio of the maxima ; and by assumption 

 ( I ) the latter is constant. Hence, even on the assumptions 

 the words italicised in the above quotation do not appear 

 justified. The southern minimum may tend towards a 

 relative increase, but there is a limit to the increase. The 

 general idea is perhaps suggestive, but the second 

 assumption involved must be considered highly impro- 

 bable, and without it the whole theory fails. 



At the end of the paper Mr. Lowell turns his attention 

 to the eccentric position of the southern cap. He 

 declines to believe that this well-known peculiarity is due 

 to local elevation, but, on the contrary, attributes it to a 

 region of depression, where the cap has acquired an 

 exceptional thickness. The phenomenon is a very 

 curious one, but speculation as to its origin has probably 

 not much value. 



THE TELAUTOGRAPH. 



IN an article on " Electric Signalling" which appeared 

 recently in these columns (vol. Ixiv, p. 6) we referred 

 to the writing telegraph invented by Mr. Foster Ritchie. 

 We have since had an opportunity of examining the 

 instrument and seeing it at work, and are enabled to 

 give a full description of it. The problem of devising an 

 apparatus which should telegraphically transmit the 

 actual handwriting or drawing of the person sending the 

 message is one which has attracted a number of in- 

 ventors. The difficulties to be overcome are, however, 

 numerous, and in consequence up to the present no really 

 satisfactory instrument has been invented. These diffi- 

 culties seem to have been mastered in Mr. Ritchie's 

 telautograph in a very ingenious manner, and the instru- 

 ment is onewhich should prove thoroughly trustworthy and 

 serviceable. Although it is not to be expected that the 

 telautograph will replace ordinary telegraphic apparatus 

 to any very marked extent, seeing that the speed of 

 signalling is necessarily limited, yet there are numerous 

 cases in which it should be of use. To give only one 

 example, there are many persons possessing private tele- 

 phone lines who would gladly supplement them with an in- 

 strument of this kind by which written instructions can be 

 sent whether there is any one present to receive them or 

 not. Other cases of like kind will doubtless occur to the 

 reader. 



One of the chief merits of the Ritchie telautograph lies 

 in the fact that only two wires are needed to connect the 



