l^OV. 21, 1877] 



NATURE 



61 



I have examined. They will come to the criticism oi my reiults 

 with fuller materials than are available to me here, but so far as 

 these materials enable me to form an opinion, the result is against 

 the existence of a common periodicity in the sun-spots and in the 

 American rainfall within the temperate zone. 



Allanton, Lanarkshire, November 4 W. W. Hunter 



Contribution to the Sun-spot Theory of Rainfall 

 The Lucknow Meteorological Observatory has been estab- 

 lished since 1868, and regular observations have been recorded 

 since that year under my superintendence. 



In Nature of December 12, 1872, Mr. Lockyer published a 

 notice of Mr. Meldrum's discovery of the coincidence between 

 the maximum and minimum sun-spot periods, and the maximum 

 and minimum rainfall in certain places. After reading it I 

 examined the annual rainfall at Lucknow from 1868 to 1872, 

 and found that there was reason to believe that the rainfall at 

 Lucknow followed the same cycle as that of the sun-spots. The 

 figures were : — 



1868 27-6 inches. 



1869 4i"9 .. 



1870 646 ,, 



1871 65-0 „ 



1872 4i'4 .. 



The equal amount of rainfall (41 inches) on both sides of the 

 maximum fall of 1870 and 1871 was very striking, and as there 

 was a rise in the rainfall from 1868 to 1870-71, and after that a 

 decrease, and having just read Meldrum's disc jvery, I conjectured 

 that the annual rainfall would continue to decrease till it reached 

 its minimum. In my annual abstract, which I submitted to 

 Government in April, 1873, and on the slender evidence of five 

 year's rainfall, I ventured to state that if Meldrum's law be true, we 

 had in Lucknow lately passed the period of maximum rainfall, 

 and were descending towards a minimum, S3 that during 1877, 

 1878, and 1879 there would be a scarcity of rain, and in one of 

 those years the minimum rainfall of the cycle would occur. I 

 am now able to give the annual rainfall of almost a complete 

 cycle, and the figures will speak for themselves : — 



1867 was a sun-spot minimum period. 



1868 27-6 



1869 419 



1870 64-6 



1871 65-0 



1872 * 4r4 



1^73 35-1 ) 



1874 5i"4 



1875 43'5 



1876 23-6 



1877 ' "7 



(Up to date October 22). 



This is October 22, 1877, and the total fall up to date has been 

 orly 117 inches, about a third of which fell in the months of 

 January, February, and March. The fall during the rainy season 

 of 1877 has been so small that great fear of a famine has been 

 felt. I considered Meldrum's discovery so important that at the 

 end of my annual abstract of meteorological observations for 

 1872, I inserted a long abstract of Mr. Lockyer's article in 

 Nature, in order to make the theory more widely known. 



I believe meteorologists are on the track of a most important 

 law. I would not expect the maximum and minimum rainfalls 

 in every place to coincide with the sun-spot maximum and mini- 

 mum so completely as that given above. Possibly in some places 

 the figures might be reversed, owing to a changed direction in 

 the water-bearing currents of the a'mosphere ; but that the 

 changes occurring in the sun have a direct influence on rainfall 

 there cannot, I think, be any doubt. E. Bonavia 



Lucknow, October 22 



Inches of 



rainfall 



in Lucknow. 



The Radiometer and its Lessons 

 I WISH that Prof. G. C. Foster had been more explicit in his 

 answer to my letter ; for as it is I cannot understand to what 

 "variations of density" he refers. So far as I know there are 

 no variations of density in the gas in question except those which 

 arise from variations of temperature ; but these variaions of 

 density certainly do not affect the rate at which heat diffuses 

 into and through the gas, for this rate is independent of the 

 density and for the same gas depend only on the absolute tem- 

 perature and on the degradation of temperature in the direction 

 in which the diffusion takes place. The variations of tempera- 

 ture do affect the rate of communication but only in proportion 



to the square root of the absolute temperature, and hence, in the 

 case of the radiometer, only to an inappreciable extent. 



It is obvious that the law of diffusion holds good only so long 

 as the gas is undisturbed by convection currents. Such currents, 

 which certainly exist, increase the rate at which heat is communi- 

 cated to the gas, that is to say, the hot surface instead of being 

 exposed to the action of still air is exposed to a wind which 

 t^nds to increase the rate of cooling. Bat the velocity of the 

 wind does not increase with the rarefaction, and the cooling effect 

 of a wind of a certain velocity does increase with the density 'of 

 the air. Hence, as I pointed out in my first paper, the motion 

 of the air will favour the force resulting from the communication 

 of heat less and less as the rarefaction is increased. 



As regards Mr. Johnstone Stoney's theory. The post which 

 brought me this week's Nature brought me also a paper from 

 Mr. Stoney, on which I venture to comment. In doing this, 

 however, I may say that I have no wish to criticise what Mr. 

 Stoney has written. The fact that Mr. Stoney has in no way 

 referred to my work, although I preceded him by some two 

 years, has relieved me from all obligation to discuss Mr. Stoney's 

 theory ; and I certainly should not do so now were it not that, 

 as Prof. Foster has instanced this theory as disproving what I 

 believe to be the truth, I feel bound either to show wherein it is 

 wrong or acknowledge rtiy inability to do so. 



In the paper which I hive just received, 1 Mr. Stoney start s 

 with an assumption that, hut for the effect of pravitation, "a flat 

 stratum o'^gas in contact with a hot surface, A," and "everywhere 

 subject to the same pressure" can exist in a state of equilibrium 

 " except at the limits," without any passage of heat from the 

 hotter to the colder part, although " within the stratum the 

 temperature gradually decreases, from within outwards, from B^ 

 the temperature of A to 0.^ the temperature of the surrounding 

 gas." 



In support of this assumpt^oa 1 cann A fin i that any prooi is offered 

 except that which is contained in the following portion of a sen- 

 tence : — "We know, from familiar experiments, which show 

 gases to be bad conductors of heat, that after the brief interval of 

 adjustment a permanent state would ensue in which there would 

 be no further change of density, or motion of heat, except by 

 radiation." 



Now this assumption and the statement in support of it — in 

 both of which Mr. Stoney seems to have ignored the very exist- 

 ence of diffusion of heat in gases — are contrary to all experience 

 as well as to the deductions from the kinetic theory of gases ; 

 for it follows directly from the kinetic theory, and has been 

 abundantly established by experiment, that under no circumstances 

 can there exist a variation in the temperature of a continuous layer 

 of gas without heat diffusing from the hotter to the cooler part. 



I think that I need say no more. This assumed condition of 

 gas forms the base of all Mr. Stoney's reasoning, and although in 

 a subsequent part of his paper he appears to me to have arrived at 

 deductions which contradict his fundamental assumption, still this 

 assumption may be held accountable for the anomalies which he 

 has found. Osborne Reynolds 



November 17 ■ ■ 



I beg to call the attention of the readers of Nature to the 

 following passage at the commencement of Mr. Crookes's lec- 

 ture at the Royal Institution on February 11, 1876, "On the 

 Mechanical Action of Light " : — 



" To generate motion has been found a characteristic common, 

 with one exception, to all the phases of physical force." [Illus- 

 trations are then given of the production of motion by heat, 

 magnetism, electricity, gravitation, sound, and chemical force.] 



"But h^At, in some respects the highest of the powers of 

 nature, /tas not hitherto been found capable of direct conversion into 

 motion; and such an exception cannot but be regarded as a singular 

 anomaly. 



"This anomaly the researches which I am about to bring 

 before you have now removed ; and, like the other forms of 

 force, light is found to be capable of direct conversion into motion, 

 and of being most delicately and accurately measured by thj 

 amount of motion thus produced." 



I cannot but suppose that Mr. Crookes and Prof. Carey 

 Foster have alike forgotten the existence of this passage. If 

 it does not convey an interpretation of the phenomena of the 

 radiometer which is now admitted on all hands to be wront.', 

 and imply a claim to the discovery of " a new mode of force," I 

 am^incapable of understanding the meaning of words, 



I may add that one after another of my eminent scientific 



I " On the Penetration of Heat across Layers of Gas," Scientific Trans- 

 actions of the Royal Dublin Society, November, 1877. 



