Feb. 6, 1879] 



NATURE 



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which give him an advantage as an observer of pheno- 

 mena both on sea and land, and the results of which are 

 apparent in the volume before us. Mr. Clark's narra- 

 tive relates to the years 1876-77, during which he resided 

 for nearly two years in Buenos Ayres, Paraguay, and 

 Uruguay. The two latter regions are yet sufficiently 

 unknown to make any contribution from a competent 

 observer who has visited them, welcome. The informa- 

 tion, especially, which he gives us on Paraguay, is of 

 much importance, and is a valuable addition to that 

 obtained by Mr. Keith Johnston, in his visit two or three 

 years ago. From a scientific point of view, perhaps the 

 most valuable portion of Mr. Clarke's book are the 

 numerous meteorological notes which he made both 

 during his voyage out and his stay in South America. 

 His knowledge of meteorology in its widest sense seems 

 to us both extensive and accurate, and his observations 

 on the instruments he used, on doldrums, tropical evapo- 

 ration, and other such topics, are really interesting. But 

 Mr. Clark knows something also of botany, as is evident 

 from the frequent observations in this direction to be 

 found throughout his volume. A whole chapter is devoted 

 to the climate and meteorology of Buenos AjTes, im- 

 portant both from a scientific and practical point of view, 

 as it is one of the great centres of emigration for South 

 America. Many interesting sketches are given of the 

 people and their mode of life in the various districts 

 visited by Mr. Clark, and altogether his work is one of 

 substantial value and real interest, and we trust it will 

 find many readers. 



Our Railways : Sketches Historical and Descriptive, with 

 Practical Information as to Fares, &»c., and a Chapter 

 on Railway Reform. By Joseph Parsloe. (London : 

 Kegan Paul and Co., 1878.) 



Mr. Parsloe's volume contains a large amount of very 

 varied information on railways, their origin, their working ; 

 its object, he tells us, being to present a sketch of our rail- 

 way system in its general details. The contents are so varied 

 it would be difficult to give any idea of their nature with- 

 out a lengthened notice. Mr. Parsloe goes back to the 

 old days of stage-coaches, coming down to the origin of 

 railways, then speaks of their construction, of navvies, 

 ^vorking expenditure, signals, gauges, tickets, and a mul- 

 titude of other topics all of much interest to the travelling 

 public. The book is certainly both interesting and 

 instructive. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken op anonymous communications. 



[The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and norvel facts.1 



The Formation of Mountains and the Secular Cooling 

 of the Earth 



The letters of Mr. Wallace and Mr. Fisher in Nature, 

 vol. xix. pp. 121, 172, 244, 267, raise the question as to whether 

 or not It is possible that the interior of the earth can be cooling 

 more rapidly than the exterior. The following is an attempt to 

 answer the query as to where the loss of temperature per unit 

 time is greatest. 



Sir W. Thomson (see Thomson and Tait, "Nat. Phil.," 

 App D) considers the cooling of " a solid extending to infinity 

 m all directions, on the supposition that at an initial epoch 

 the temperature has had two different constant values on the 

 two sides of a certain infinite plane." The solution given is— 



V = Vo + 



2V 





dz 



where k denotes the conductivity of the solid, measured in 

 terms of the thermal capacity of the unit of bulk ; 

 V, half the difference of the two initial temperatures ; 



v^, their arithmetical mean ; 



/, the time ; 



X, the distance of any point from the middle plane j 



V, the temperature of the point x at time t. 



The above solution shows that for all values of the time when 

 X = o, V = v^, so that the temperature at the medial plane is 

 constant. 



Then differentiating v with regard to the time we have — 



- jr' 



- ^ = ^ £ ;T*7 

 dt ^Slifk t^' 



This expression is that required for the rate of cooling. We 

 now wish to find where it is a maximum. Consider the function 

 ze~ ^^ ; this is clearly a maximum when log s - s- is a maximum, 



and by the ordinary rules this is a maximum when — = 2 z, or 



z 

 when s^ = \. 



Hence it follows that - — has its TnaTimnin value where 



zkt. 



dt 



Now when the unit of leng th is a foot and of tune a year, 

 k - 400 ; hence x = s/Zoot. 



This formula shows that the seat of the maximum rate of 

 cooling moves inwards as the time increases. If the time which 

 has elapsed from the initial state be two hundred million years, 

 or 2* = 2 X 10^, we have x — 400,000 feet, or a little less than 

 eighty miles. 



Sir W. Thomson shows, in his paper on the Secular Cooling 

 of the Earth, that the solution of his ideal problem will be very 

 nearly correct for the case of the earth, which is supposed to be 

 a hot sphere cooling by radiation. 



It follows, therefore, from the numerical result which i> given 

 above that the seat of the maximum rate of cooling must 

 probably be something like ico miles below the earth's s\irface. 



It does not, of course, necessarily follow that the seat of the 

 maximum rate of contraction of volume should be identical with 

 that of the maximum rate of cooling ; yet it seems probable that 

 it would not be very far removed from it. 



The Rev. O. Fisher very justly remarks that the more rapid 

 contraction of t>e internal than the external strata would cause 

 a wrinkling of the surface, although he does not admit that this 

 can be the sole cause of geological distortion. The fact that the 

 region of maximum rate of cooling is so near to the surface 

 recalls the interesting series of experiments recently made by M. 

 Favre (of which an account appeared in Nature, vol. xix. 

 p. 103), where all the phenomena of geological contortion were 

 reproduced in a layer of clay placed on a stretched india-rubber 

 membrane, which was afterwards allowed to contract. Does it 

 not seem possible that Mr. Fisher may have under-estimated the 

 contractibility of rock in cooling, and that this is the sole cause 

 of geological contortion ? G. H. Darwin 



Storm Warnings 



A New York telegram occasionally annotmces that a cyclonic 

 storm will probably reach the coast of Europe in a few days. 



Such warnings are often of great value ; but many storms are 

 deflected in the Atlantic, while others — without having touched 

 the American coast — come unannounced with destructive 

 violence. 



A floating buoy might be constructed to serve the purpose of 

 a marine observatory, when placed in the usual track of storms 

 at a sufficient distance from exposed coasts to be useful for 

 warnings for ships in and near harbours. 



The chief meteorological "elements" which are of essential 

 significance in such a case are the hsiqht and changes of the 

 barometer, and the varj'ing force and direction of the wind. 



If an experimental buoy were fixed by means of a slightly elastic 

 cable about eighty miles off Valencia Observatory, and con- 

 nected therewith by submarine telegraph wire, a slight modifica- 

 tion of the aneroid lodged therein would enable the observer 

 on shore to determine to about a tenth of an inch the height 

 and changes in its readings. 



A wind-vane in connection with a magnetic bar, and present- 

 ing a disk to the air-current, might be made the means of regis- 

 tering approximately the force and direction of the wind. 



