596 



NATURE 



[October 20, 1898 



not describe experiments (of which I have made plenty) in proof 

 of its occurrence. 



(8) I quite agree with Mr. Cunningham and Mr. Henslow, that 

 it is my duty to describe the effect I believe fine mud to have 

 upon the respiratory apparatus, and I am preparing such a 

 description as quickly as I can. I hope also to be able before 

 long to answer Mr. Cunningham's last and very pertinent ques- 

 tion, whether crabs of given length, from the clear water outside 

 Plymouth Sound, are broader or narrower than crabs of the 

 same length from muddy waters within the Sound. 



(9) I altogether fail to understand Mr. Henslow's letter, and 

 I fear that my imperfect exposition has led him to misunder- 

 stand me as completely as he has misunderstood one of the 

 clearest passages in the " Origin of Species." Mr. Henslow 

 suggests that a variation, fit to afford material for natural 

 selection, must be a new character, differing in some mysterious 

 and undefined way from those individual differences which he 

 refuses to call variations ; and he further attributes the same view 

 to Mr. Darwin. If Mr. Henslow will read once more the section 

 of the fourth chapter of the "Origin of Species " headed " Il- 

 lustrations of the Action of Natural Selection, &c.," he will see 

 that Mr. Darwin does not express this opinion. The important 

 thing to determine is not what any man, however eminent, has 

 said about the importance of differences between individual 

 animals, but what that importance can be shown to be. The 

 crabs at Plymouth have not, during the past five years, exhibited 

 any changes in the magnitude of their frontal breadth which Mr. 

 Henslow would rank as a variation, but they have exhibited in- 

 dividual differences. During these five years the mean frontal 

 breadth ratio has changed nearly 2 per cent., so that the change 

 now going on would produce, if it were to continue at the same 

 rate for fifty years, a change big enough to constitute a difference 

 which most men would rank as specific. I have endeavoured 

 to show that this change has been accompanied by a destruction 

 which has acted selectively upon individual differences. Mr. 

 Henslow has not seriously discussed this attempt of mine, but 

 ridicules the idea that so small a change can be of importance in 

 relation to evolution. If the mean stature of Englishmen were 

 to diminish by an inch in a few years, I presume Mr. Henslow 

 would regard such change as rapid and important ; but the per- 

 centage change would be less than that which Mr. Thompson 

 and I have demonstrated during the past five years in crabs. 



W. F. R. Weldon. 



Mirage on City Pavements. 



During my summer visits to San Francisco, I have been so 

 frequently struck with the beautiful miniature mirages that can 

 be seen on the flagstone sidewalks whenever the sun shines, 

 that I determined to secure, if possible, a photograph of the 

 phenomenon on a scale suitable for reproduction. One or two 



walk is flooded with a perfectly smooth sheet of water, in which 

 the reflections of pedestrians can be seen as distinctly as in a 

 mirror. 



In order to observe the phenomenon it is necessary that a 

 considerable stretch of level pavement be foreshortened into a 

 very narrow strip. This is the condition in the photograph : 

 the camera stood just below the brow of the hill, and 

 the distance in the photograph from the X to where the 

 children and the toy cart are standing, is an entire block (135 

 yards). The position of the camera and section of the hill-top 

 are shown in the diagram. The apparent reflections, due to the 



bending upward of the rays by the thin layer of heated air, 

 come out very clearly in the picture, but the camera fails to give 

 a correct reproduction of the extreme brilliancy of the reflecting 

 layer of air. 



On taking a few steps up the hill, decreasing the foreshorten- 

 ing, the glaze vanishes, and we see only the dull grey of the flag- 

 stones. Extremely hot sunshine is not necessary. I have 

 observed the phenomenon early in the morning after a cold 

 night, before the sun had reached the pavement, the slight 

 warmth from the ground being sufificient. Under these con- 

 ditions, however, the pavement must be more foreshortened than 

 when in the full sunshine. The refracting layer is probably only 

 a thin skin of warm air, which adheres as it were to the surface 

 of the flagstones, for the mirage is unaftecled by the strong 

 winds which frequently sweep the top of the hill. 



Probably these mirages can be seen on any level pavement 

 where the eye can be brought into the proper position. 



Physical Department of the University, R. W. Wood. 



Madison, Wisconsin, September 20. 



previous attempts in past years having been partial failures 

 owing to the smallness of the image, I secured, through the 

 kindness of a friend, the use of a very fine tele-objective capable 

 of giving an image six or eight times as large as an ordinary 

 objective of 12 inches focus. The streets over some of the hills 

 are so laid out that it is possible, on nearing the brow, to bring 

 the eye on the level of the side-walk, and look along a perfectly 

 level_ stretch of one hundred yards or more. Standing in this 

 position it is almost impossible to resist the conviction that the 



NO. 151 2 VOL. 58] 



Transference of Heat in Cooled Metal. 



My attention has just been called to two communications to 

 your journal, entitled "Transference of Heat in Cooled Metal." 

 The first, by M. Henry Bourget, appears in the issue of June 

 30, and the second, by Mr. Albert T. Bartlett, in the issue of 

 September i. 



About the year 1880 I had occasion to heat one end of an iron 

 bar to a bright red heat whilst 

 holding the cooler end in my 

 hand. Upon plunging the heated 

 end into a bucket of water the 

 cooler end became suddenly so 

 hot that I was obliged to release 



♦ my hold on it. 



• This phenomenon interested 



me very much, as I could find 

 no explanation for the apparent 

 reflection of heat to the cooler 

 j end of the bar ; and in 1888, 

 A whilst working in the physical 



*► laboratory at Johns Hopkins 



University, I further investigated 

 the matter. 



To one end of an iron or 

 steel bar was soldered a thermo- 

 electric couple, the circuit of 

 which was closed through a very 

 sensitive, high resistance, Rowland, reflecting, galvanometer. 

 The bar was passed through two pasteboard screens, and was 

 supported in a horizontal position, the screens serving to inter- 

 cept any heat which might be conveyed by radiation or con- 

 vection through the air from one end of the bar to the other. 

 Under the end of the bar, remote from that to which the 

 thermo-electric couple was soldered, was placed a compound 

 bunsen burner, by which the end of the bar was raised to a dull 

 red heat. The spot of light on the galvanometric scale 



