40 



KNOWLEDGE. 



[Febbuaky 1, 1901. 



and drier? I would invite atteution to the following 

 facts in this relation : — 



1. The last 30 years (1871-1900) contain a smaller 

 number of days with jnaximum temperature, 70" or more, 

 than any other 30 consecutive years since 1841. (Of 

 such groups there are obviously ol.) 



Curve A shows the general decline in those values, 

 The first year-point re]>resents the number of those hot. 

 days in the 30 years ending 1870, the next in the 30 years 

 ending 1871, "and so on. The difference between the 

 highest aud the lowest point is 2470 - 2273 = 197. 



Next, let us consider the number of wet months in the 

 summer half [April to September) in 30 years ending 

 1870, '71, &c. We have the curve B, showing also a 

 general decline. We may say this about it : 



2. In each of the 30-ycar groups ending 1898 and '99, 

 the number of those wet months was less tliau in any 

 other group (than these twol of 30 consecutive years since 

 1841.' (The value for 1900 shows a slight rise.) The 

 diiference between the highest and the lowest value is 

 89-72 = 17. 



I do not represent that this gradual decline is likely to 

 continue. More proViably we shall ere long find compen- 



iss fjd '3 %'cj 'fo^r'y '<^( yj f?^ 



A. Uiivs with maximum temiioniture 70" or more in 30 years ending 



1870, '71, ete. 



B. Wet montLs in summer lialf in 30 years ending 1870, '71, cte. 



sation, aud a rise in the curves. It seems desirable to get 

 light on the limits aud cause of those variations. 



Alex. B. MacDowall. 



RAINBOW PHENOMENA. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — The double rainbow observed by Mr. S. R. S. 

 Brown at Eastbourne, and recorded in Knowledge for 

 December, 1900, must have been a very good example of 

 what ai-e called " supemumerai-y " bows, and which are 

 said to be due to interference {see Scott's " Elementary 



Meteorology," p. 200, 1st Edition), but I would like 

 to see the mathematical proof of this. 



I have often noticed these supernumeraries,'but never 

 more than three inside the primaa-y bow. It would 

 be interesting to know if any one has ever observed the 

 same thing in connection with the secondary bow. 

 Sometimes the bow has been nearly tlie complete semi- 

 circle, and at others and more commonly only a very 

 small segment of the bow was projected above the 

 horizon. One especially which I observed in Esses a 

 good many years ago was very brilliant, and the third 

 or inmost supernumerary only just cleared the horizon. 

 I fancy they are always present and to be seen provided 

 the sun's rays are br-ight enough and tlie background 

 of cloud is dai'k enough. 



On the 26th September last I saw a. rainbow which 

 besides exhibiting the supeniiuucraiy bows had another 

 variation, which I described as follows in my weather 

 recwd : — " The bow seen at 5.30 p.m. showed inside the 

 ordinaiy primary, and close to it two supernumerary or 

 interference bows ; at about half the height of the bow 

 a short piece of the supernumeraiy bows was broken 

 from the circular baud of the bows, and stood at an 

 angle of 15° to 20° from the circle of the bows. The 

 whole bow was standing against the streaky cloud rays 

 which accompanied a shower that had just passed over, 

 and these sti'eaks seemed to be in some degree parallel 

 to the broken piece of the supernumeraries." 



As to the cause of this there must have been a re- 

 flection of the sun's rays in some way. 



I should add for the further inforniation of those 

 interested that the ground to the west rises gradually, 

 and that the clouds on which the bow was projected 

 were over Lake Temiscamingue, which gave me a good 

 view of the phenomenon. Paul A. Cobbold. 



Haileybury, Out., Canada, 

 December 16th, 1900. 



'IS HUMAN LIFE POSSIBLE ON OTHER 

 PLANETS?" . 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — Your correspondent A. D. Taylor enquires in 

 the January number of Knowledge as to whether those 

 who have made a study of the 2'lanets can throw any 

 light on the above subject. He especially calls our atten- 

 tion to the planet Mars, a planet which he has hithertx> 

 considered suitable for the maintenance of human life. 

 In passing rapidly over these controverted subjects 

 we may say that the greater part of tlic surface of this 

 planet is desert, that the water supply is veiy scanty,— 

 the greenish looking patches on the planet, which 

 have hitlierto been considered seas, are, accoixliug to 

 Mr. Percival Lowell, nothing but large tracts of vcge^ 

 tation gi-owing in the bottoms of the old s as. The 

 inhabitants, if such there were, would be dependent for 

 their water supply from the annual melting of the polar 

 snow or hoar-frost. The atmosphere is thin, and conse- 

 quently free from clouds ; it is doubtful whether there 

 are any rainfalls on the planet, the m^oisture being 

 deposited in the form of dew or hoai'-frost. The question 

 of the composition of the Martian atmosphere is of the 

 greatest importance. Your readers are aware that the 

 chemical composition of any atmosiDliere is dependent 

 on the critical temperatiu-e and the critical velocity of 

 its gases. In other words, if the gi-avitational pull of 

 a planet is not greater than the critical velocity of its 

 gases, the molecules of gas will leave the planet never 

 to retm-n. In the case of Jupiter, we find the atmosphere 

 is exceedingly dense, while such bodies as Mercury 



