5o6 



NATURE 



\Oct. II, 1877 



irregulariiies for each station which almost disappear 

 when the mean of the three is taken. 



1869 1870 1871 1872 1873 1874 1875 1876 



Chakra'a ... 606 57 6 658 616 642 dyo 68°3 65°5 



Roorkee ... 51-4 388 428 50Z 539 50-6 5tl 537 



Lucknow ... 440 430 450 487 47-8 51 •! 498 53-6 



Mean ... 52-0 465 51-2 535 553 562 56-4 576 

 Physicists appear to agree in the opinion that the tempe- 

 rature of parts of the solar atmosphere, as indicated by 

 the great outbursts of hydrogen and other well-known 

 phenomena, must be highest about the time of maximum 

 spot-area ; but I think the above figures will show that 

 the question whether the amount of radiation which 

 escapes into space is then at a maximum or not is still an 

 open one. It must, at least, be admitted that the relative 

 darkness of the spots is an indication of low temperature 

 and consequent absorption. The registers of the Indian 

 meteorological stations during the next ten years will 

 probably give the data for determining the question, all 

 the solar thermometers employed since the beginning of 

 the present year having been carefully compared by 

 exposing them side by side with an arbitrary standard 

 before they were issued to the stations. The readings of 

 one year will therefore be strictly comparable with those 

 of another, notwithstanding the fact that breakages 

 frequently occur. 



It will probably be said that the very fact, now pretty 

 well established, that rainfall is greatest in maximum 

 sun-spot years argues increased evaporation and increased 

 solar radiation during those years. The stations whose 

 rainfall returns have been examined by Mr. Meldrum 

 and those others who have worked at the subject are, 

 however, not by any means unilormly distributed over 

 the earth. The great majority of them are situated within 

 or near the tropics, or in the maritime districts of tem- 

 perate regions, and their more abundant rainfall in 

 maximum sun-spot years might be easily explained by 

 the diminished carrying power of the winds at that epoch 

 of the solar cycle. Prof. Koppen has shown that the 

 periods of maximum and minimum terrestrial temperature 

 coincide approximately with the minima and maxima of 

 sun-spots, and that both the maximum and minimum 

 annual temperatures are reached somewhat so:ner in the 

 tropics than in the temperate zones. One would think, 

 therefore, that the great convection currents of the atmo- 

 sphere, depending on differences of temperature, would 

 be least powerful a little after tiie maximum of sun-spots 

 which is the period of heaviest tropical rainfall, and blow 

 most strongly after the sun-spot minimum, the period of 

 least rainfall in the tropics. The only data I know which 

 would enable one to form an estimate of the rainfall of a 

 large inland area in the temperate zone during a long 

 term of years, are embodied in a diagram of the fluctua- 

 tions of level of the North American lakes, given in a 

 paper by Mr. G. M. Dawson, in Nature, vol. ix. p. 506. 

 The diagram shows a remarkable coincidence between 

 the variaiions of the level of the lakes and those of the 

 sun-spot area, and the inference Mr. Dawson draws from 

 a comparison of the two curves appears to be that high 

 water in the lakes is the result of great solar activity when 

 the spot-area is large. Since the appearance of this 

 paper in 1874, it does not appear to have struck any of 

 the readers of Na'IURE that, in every instance except one, 

 high water in the lakes preceded the sun-spot maximum 

 by two or three years, and, in like manner, the lowest 

 level was reached several years before the sun-spot 

 minimum ; but a reference to the diagram will show that 

 such IS the case. It is evident, therefore, that high water 

 in the lakes cannot be a consequence of numerous sun- 

 spots, but it may be a commutative effect of greater eva- 

 poration than usual and greater carrying power in the 

 winds during the few years of high temperature succeeding 

 the sun-spot minimum. 



I have not at hand any means of ascertaining the rela- 

 tive velocities of the wind at European stations during 

 the years about the maximum and minimum epochs of 

 the solar cycle, but the anemometer records of Indian 

 stations shows that the wind velocity varies directly with 

 the temperature. The following table gives the mean 

 velocity each year at five stations, as measured by a 

 Robinson's small anemometer. The figures represent 

 miles per diem : — 



Calcutta ... 



Hazaribagh 



Benares 



Agra 



Bareilly 



1869 1870 1871 1872 1873 1874 187s J876 



— — 122 125 133 140 120 133 



— — 128 140 157 :6o 172 188 

 79 58 54 68 74 93 116 128 



121 180 100 94 86 97 102 103 



124 114 89 64 67 72 75 — 



If the yearly average for the first four stations be taken, 

 it will be seen that there is a regular increase in the 

 velocity of the wind from 1871, the probable year of 

 lowest terrestrial temperature, to 1876, which was probably 

 the hottest year of the period. It is, therefore, I think, 

 at least possible that the excess of tropical and oceanic 

 rainfall in maximum sun-spot years may be caused by 

 precipitation near the place of evaporation, owing to the 

 diminished force of the trade-v/inds and anti-trades at 

 those periods, and that if the winter rainfall of Europe 

 and America were examined, it might show an excess in 

 ?ninimtim sun-spot years, derived from vapour brought 

 by an unusually strong upper current from regions of 

 great evaporation in the South Atlantic. 



The registers of nearly twenty years show that 

 the winter rainfall of India, north of the tropic, is 

 probably subject to such a periodic variation, and if 

 this surmise be verified in the future it may prove 

 to be of the greatest economic importance. Last 

 cold weather these rains were unusually abundant, 

 and enabled the cultivators of Northern India to grow a 

 spring crop sufficient not only for their own wants, but 

 for export to Europe as well as to the famine-stricken 

 districts of Madras and Bombay. Both this year and 

 last the regular summer rains have been far below the 

 average, and almost any day since last June the vapour 

 that in an ordinary year would have come down to fertilise 

 the soil might have been seen passing overhead in the 

 form of light cirrus drifted by a strong south-west wind. 

 The moist easterly current from the Bay of Bengal, from 

 which a large proportion of the rainfall of the Gangetic 

 valley is generally derived, has this year scarcely pene- 

 trated as far west as Benares. The natural consequence 

 of this failure of the rains will be a famine in Northern 

 India, unless, next cold weather, we get the heavy rains 

 which the experience of past years leads us to expect. 



A comparison of the mean temperature, vapour tension, 

 humidity, and rainfall for the month of July in the years 

 1875, 1S76, and 1877, shows that the extraordinary dry- 

 ness of the present year is the result not so much of the 

 absence of aqueous vapour from even the lowest stratum 

 of the atmosphere as of the abnormally high temperature 

 which prevents its precipitation. This will be seen from 

 the following table : — 



July, 1 875 

 ,, 1876 

 „ 1877 



81-4 

 85-4 

 903 



•984 

 •897 

 ■862 



Humidity. Rainfall. 



per cent. inches. 



.. 84 ... 26-33 



.. 75 ... IOCS 



.. 63 ... 2-23 



The figures in the table are deduced from observations 

 taken at Allahabad four times daily, viz., at 10 and 4 A.M. 

 and P.M. The year 1875 was marked by unusually heavy 

 local rains in July which laid a great part of the surround- 

 ing district under water, so a comparison of 1876 and 1877 

 with 1S75 is hardly fair. The average rainfall of the 

 month is 14 65 inches. S. A. Hill. 



Allahabad, August 29 



