114 



KNOWLEDGE 



[June 1, 1892. 



expect that the northern or continental slopes of the 

 Himalaya Mountains would be colder but also drier than 

 the southern ones, which are exposed to the ocean. Yet 

 upon the warm southern slopes we find the line of perpetual 

 snow considerably lower than upon the colder northern 

 ones. Thus, because the supposed snow-caps upon Mars 

 are small, it does not necessarily indicate that the tem- 

 perature of the planet is higher than that of the earth. 



William H. Pickering. 



Arequipa, Peru, April 14th, 1892. 



[Prof. AV. H. Pickering's interesting suggestion, that as 

 a planet cools its oceans will be absorbed and go to 

 augment the volume of its underground waters, does not 

 commend itself to my mind as explaining the phenomena 

 we observe on Mars. For, in order that the earth should 

 absorb say a mile in depth of its ocean covering, the 

 isothermal surface within the earth, at which water is 

 converted into steam, would need to fall at least three or 

 four miles, and all other isothermal surfaces would fall 

 with it ; that is, the mean temperature of the surface, even 

 in the equatorial regions, would certainly fall far below the 

 freezing point. But this does not seem to be the case on 

 Mars. 



If we assume, as most speculative astronomers have 

 hitherto assumed, that the white polar caps of Mars 

 are due to snow, it follows that the mean temperature 

 of the Martian surface in the equatorial and temperate 

 regions of the planet must be above 32° Fahr., a fact 

 which is not consistent with the assumption so fre- 

 quently made, that the mean temperature of a planetary 

 surface must vary inversely as the square of the distance 

 of the planet from the sun. We are, it seems to 

 me, forced to assume either that the polar caps of Mars 

 are not due to snow, or that the mean temperature of the 

 equatorial and temperate regions of Mars is above 32° 

 Fahr. ; that is, that they are much warmer than they 

 should be on the assumption that the mean temperature 

 will vary inversely as the square of the distance from the 

 sun. To my mind, the discrepancy is most easily 

 accounted for by supposing that the atmosphere of Mars is 

 more dense than the atmosphere of the earth. I do not 

 see any necessity for the assumption that the amount of 

 the atmosphere and the amount of the ocean on Mars 

 must bear the same proportion to the amount of solid 

 material as on the earth, or even that they must have 

 originally borne the same proportion. It is, of course, 

 possible that we may be observing phenomena on Mars 

 quite dissimilar to those going on here. The white polar 

 caps may be due to the deposition of the snow-like crystals 

 of carbonic acid, which evaporates again at a temperature 

 far below the greatest cold which we experience on the 

 earth's surface. — A. C. Eanyard.^ 



A CORRECTION. 

 To the Editor of Knowledge. 



Dear Sir, — My attention has been called to an error in 

 the (leiiratinn of the expression for the tangential force in 

 the article on the " Origin of Binary Stars," and I gladly 

 seize the earliest opportunity for correcting the mistake. 

 The height of the tide obviously varies as the fiixt poiirr 

 of the tide-generating force, not as the xiiuarc, as asserted 

 near the bottom of the first column on page 82. The 

 couple acting against the rotation of Helios arises from 

 the excess of the attraction of Sol on the nearer tidal pro- 

 tuberance above that on the further. Now this excess is 

 found to vary inversely as the third power of the distance 

 between the two bodies. But the couple also varies 

 directly as the height of the protuberance {i.e., as the 



height of the tide), and this height varies inversely as the 

 third power of the distance. Hence the tidal frictional 

 couple varies as the inverse sixth power of the distance ; 

 or it may be described as varying inversely as the square 

 of the tide-generating force, since the tide-generating force 

 varies as the inverse cube of the distance. If we denote 

 the tidal frictional couple by T, the radius vector by fi, 

 and the tangential force by t, the principle of action and 

 reaction gives for the equilibrium of the forces fp = T, 



rp 



In the fifth line from the top of page 82 



or t = 



P P' 

 (second column 



sixth power.' 



the reading should be " cube " instead of 

 Since the result reached in the article 

 was correct, the error in the derirntion of the expression 

 for the tangential force does not vitiate my argument, and 

 the conclusions are therefore sound. But as a friend had 

 kindly called my attention to the slip, I thought it ought 

 to be corrected. Very faithfully yours, 



Berhn, May 20th, 1892. T. .T. -T. See. 



CAN A PLANET BE FINALLY SURFACED WITHOUT 

 A SEDIMENTARY ROCK SERIES? 



■ To the Editor of Knowxedge. 



Dear Sir, — Professor -ludd, in his instructive work on 

 Volcanoes, at page 305, tells us that " the Moon ai')pears 

 to be destitute of atmosphere and water . . . under the 

 circumstances we find its surface, as we might expect, to 

 be composed of rocks which appear to be entirely of igneous 

 origin " ; and at page 367 he refers to the striking evidence, 

 on the Moon, of the action of volcanic forces, in the vast 

 size of the so-called "craters" (up to 50 and CO miles 

 diameter). 



But inasmuch as Professor Judd's entire work is a 

 peculiarly good demonstration that " without water, there 

 can be no volcano," I should esteem it a great favour if 

 any of your scientific readers can explain the obvious con- 

 tradiction in the above. 



For over ten years I have carefully studied the lunar 

 surface, in the hope of eventually solving this well-known 

 astronomical riddle, and have come to the conclusion that 

 such a globe cannot have been finally surfaced (solely) by 

 igneous and volcanic agency. 



The convection and radiation of heat from rock 

 surfaces, as the initial temperature declined, must have 

 been such an exceedingly slow process that it is incon- 

 ceivable how a globe of that size can have passed from a 

 semi-molten to the airless and waterless stage of develop- 

 ment, without the intervention of a very prolonged era of 

 erosive denudation. 



Selenographers are agreed that on our moon there is 

 practically an absence of river valleys, and the fluviatile 

 sculpturing so characteristic of our earth's surface ; in 

 fact, that the lunar surfacing seems to be due entirely to a 

 deposition of solid dry material, showing a singular and 

 marked absence of all drainage phenomena. 



Hence it is urged that there is an absence of sedimen- 

 tary stratified rocks, what we see being the unchanged 

 results of primeval lava lakes, volcanic outbursts, and 

 " cinder rings," laid during an era of such high initial 

 temperature that water, near the surface, would have been 

 a physical impossibility. 



The subsequent decline in temperature, again, is assumed 

 to have been so exceedingly rapid as to preclude the possi- 

 bility of any erosion of river valleys, and the formation of 

 sedimentary rocks. 



An idea prevails that the enormous craters and volcanoes 

 (seen even all over the poles) originated during the period 

 of high initial temperature, and that the cavities, 20 and 



