February 7, 1895 J 



NATURE 



541 



•crystal, they often serve to determine the particular type 

 ■of symmetry to which the structure belongs when 

 polarised light may merely indicate whether the crystal 

 is op'ically isotropic,uniaxal, or biaxal. Dr. liiumhauer 

 has done a greit service by preparing for publication 

 a series of mignified phjtographs of the figures 

 produced by etching different faces of crystals of 

 the following subu incei :^FIuor, Blende, Cryolite, 

 Apatite, Nepheline, Datolite, Zinnw ildite, Le;icite, Bora- 

 cite, Dolomue, M ignesite, Chalybite, Nickel sulphate, 

 Strychnine sulphate. 



The figure on the previous page shows the hexagon- 

 ality of the depressions which, alter etching, are seen on 

 41 basal plane of a crystal of Apitite. 



These beauiifully executed photographs 'are accom- 

 panied by detailed descriptions (85 pp.), and by a sketch 

 of the history and development of the methods ("46 pp.). 

 Dr. Baumhauer has done such excellt-nt work in this 

 branch ot science, that he is parti ularly qualified to 

 select and describe examples which illustrate the poten- 

 tiality of this mode o( investigating crystalline structure. 

 The photographs are mounted on twelve separate plates, 

 so that they may be conveniently used in the class- 

 room. 



Suiinner Studies of Birds and Bonks. By '.V. Warde 

 Fowler. Pp. 288. (London: Macmillan and Co., 

 1895.) 

 Forest Birds ; their Haunts and Habits. By Harry F. 

 Witherby. Pp. 98. (London : Kegan Paul, Trench, 

 Tiiibner, and Co., 1 89+.) 



It would be difficult to find a worthier disciple of Gilbert 

 White than Mr. Warde Fowler. Many of While's most 

 exact and enduring ob5ervations were made on birds, 

 and the " Natural History of Selborne " has furnished his 

 imitators with descriptions of the characteristics of the 

 Stone-curlew, the Ring-ousel, the House-martin, the 

 Sand-martin, the Goat-sucker, and many more of the 

 birds of which he recorded the habits and movements. 

 Hut Mr. Fowler's writings are not mere paraphrases of 

 his prototypes. He is gifted with the " nice observation 

 and discernment '' required by every student of nature; 

 and he can express himself in attractive language — 

 attractive because it is unaffected, and because it is devoid 

 of thin sentiment and gush. The result is that the pspers 

 reprinted in this volume are reahy valuable contributions 

 to ornithology. To remark that there is not a dull page 

 in the book may be a trite saying; nevertheless, so far as 

 I we are concerned, it is a true one in this case. What 

 1 could be more readable than the chapters on the birds of 

 the Engstlen Alp, and the birds of Wales : and where 

 will you t^.nd a better example of careful and painstaking 

 observation than is afforded by the account of the 

 Marsh Warbler in Oxfordshire and Switzerland? 

 The chapter on the songs of birds is most interest- 

 ing ; that descriptive of Aristotle's writings on birds, 

 (in which fact and fiction are given equal prominence), 

 should be read by all naturalists ; and the bingrnphy of 

 Gilbert White shows that the author is saturated with the 

 spirit of the naturalist whose work has had such a wide 

 influence upon natural history during the last hundred 

 years. 



Little more need be said about this charming book. So 

 many volumes on popular natural history have lately 

 appeared, that the subject has probably begun to pal! 

 upon the public taste. Mr. Warde Fowler's work, how- 

 ever, is so full of interest ; it breathes out the air of the 

 fields and streams so pleasantly, that it carries its own 

 welcome to the heart of every lover of nature. 



The second of the two books of which the titles are 

 given above, is of quite a dilTerent character from the 

 first. It is remarkably well illustrated, is daintily bound, 

 and is nicely printed, and therefore it forms an attrac- 

 tive volume, so far as appearances go. But the text is 



NO. 1319, VOL. 51] 



dull ; for the author, while possessing the essential love 

 of nature, lacks the words with which to express it 

 eloquently. To him, "the Stork Dove is f.urteen 

 inclies in length from the tip of the beak to the end of 

 the tail, and its stretch of wing is twenty-six inches. Its 

 general colour is bluish-grey. The head, wings, and 

 back are of this colour, and the tail is the same, but 

 tipped with leaden grey." There is much more of the 

 same kind in the book ; and we confess that after read- 

 ing Mr. Warde Fowler's smooth phrases, Mr. Wuherby's 

 composition appears spiritless. The illustrations, how- 

 ever, are g )oJ e lou ^h to m ike up for deficiencies of the 

 text, and for their sake alone, the book is worth buying. 



LETTERS TO THE EDITOR. 



\_The Editor tioei not hold himself responsible for opinions ex- 

 pressed by his correspondents. N.ither can he undertake 

 to return, or to corresponl with the writers of rejected 

 manuscripts intended for this or any other part of Na I'URE. 

 No notice is tat:en of anonymous communications.] 



On the Age of the Earth. 



In reference to Lord Kelvin's leiter (Nature, January 3, 

 p. 227), he will, no doul)t, presently communicate »i h you on 

 the results of measurements which he is miking on conductivity 

 and temperature in rocks. I do not wish 10 foicslall bim by 

 referring at present to his more recent correspondence wiih me. 

 Three weeks ago I sent him the solution of the problem of a 

 cooling sphere whose cinductivity i and volumeiric capacity c 

 are any functions whatsoever of the temperature z; hut which 

 are always pr>p jrtional to one ano'her. As Mr. lleaviside 

 hal been writing to me, and had shown me that under certain 

 circumstances the differential equation became linear, and as I 

 had used his operators much a."; he himself uses the n, I cannot 

 say to what extent I cm claim credit for the work. I now venture 

 to stnJ you the more general case. 



If 



/' 



dv = K 



(I) 



so that » is the total amount of sensible heat in unit volume. 

 The general equation is 



^^(h'^-^\ + l(h±\ + !L/i'^'!\ = '^" 



{(h'^l\ + l(i±\ + !L(i'^^] = ^' . . . (2) 

 x\ dx) dy\ dy) d\ dz) dt 



Now let k = CK where k is a constant, then 



and the equation becomes 



dit du 



K — as <r = 

 dv dv' 



d'^u dht d'^u _ I du 

 dx- dy- dz- k dt 



.(3) 



Any of the many problems that have been worked out oB 

 the distribution of u may be translated in'o temperature pro- 

 blems. Thus in a body before / = o let k = k, a constant 

 everywhere, and after / = o let the suitace have eveijAheie a 

 constant a = h„. Let ihe solution be 



« = («, - H,)F (X, y,z,t) .... (4) 



In the boiy before / = o let w = o, a constant everywhere, 

 and alter ? — o let the >urface have everywhere v - Vi, a con- 

 stant, then if ?/is the temperature anywhere 



. . .(5) 



jc . 'lv = (h, - Ho) / (x, y, z, I) 



so that a table of the values of (r . r/o enables the tempera- 

 ture at any place to be calculated. 



If - the rate of increase of u normally inward from the 

 dn 

 surface at any place, has been calculated, say that it is 



du 



dn 



= («, - «o)F(-r, J-, s, ') 



(6) 



