NOVEMBKR 13, 1903.] 



SCIENCE. 



G21 



ing currents, and thus reducing dynamically the 

 pressure of the atmosphere as a whole. The first 

 and second of these processes do not indeed di- 

 rectly reduce the pressure but only the density 

 of the air stratum while they increase its volume. 

 In order, therefore, that the observed effect may 

 follow, a portion of the higher atmosphere must 

 be removed, and this will necessarily flow away 

 to regions where the production of vapor is at a 

 minimum, viz., the polar and cooler portions of 

 the temperature zones, and more especially those 

 where a cold dry land surface radiates rapidly 

 under a winter sky. Such an expanse is the 

 great northern plain of European Russia and 

 Western Siberia north of the Altai. 



In 1886 we got the first fruits of the ob- 

 servations of the widened lines in sun 

 spots, which had been obtained on a def- 

 inite plan, since 1879. The changes which 

 occurred from a spot-minimum to a spot- 

 maximum, and some distance beyond, had 

 therefore been recorded. The changes 

 were most marked, showing a great change 

 in the chemistry of the spots at these times. 

 At minimum the lines chiefly widened were 

 those of iron and some other metals, but 

 at the maximum the lines widened were 

 classed as ' unknown, ' because they had not 

 been recorded in the spectra of the ter- 

 restrial elements. It was reasonable to 

 suppose, therefore, that the sun was not 

 only hotter at maximum, but hot enough to 

 dissociate iron vapors.* 



In 1891 Janssen's suggestion of 1869 

 was brought into a practical shape for 

 observatory work, by Hale and Deslan- 

 dres,t and the prominences on the sun's 

 disc, and surrounding it, were photo- 

 graphed in full daylight by using only the 

 light radiated by the calcium vapor, which 

 they always contain. 



By the year 1900 we had acciunulated, 

 at South Kensington, observations of the 

 widened lines for a period of over 20 years. 

 There was a curious break in the regularity 

 of the results obtained after 1894, and the 



' Proc. Royal Soc, 1886, p. 353. 

 fComptes Rcndus, August 17, 1891. 



Indian meteorologists reported contem- 

 poraneous irregularities in the Indian rain- 

 fall, 



I determined, therefore, to make a con- 

 nected enquiry into both these classes of 

 phenomena. Thanks to the establishment 

 of the Indian Meteorological Department 

 in 1875 we had rainfall tables extending 

 over a quarter of a century, and in the 

 tropics, where the problems might be taken 

 as of the simplest, to compare with the 

 new solar data. 



I have already stated that in the pre- 

 liminary discussion of the most widened 

 lines observed in the sun spots up to the 

 year 1885, a most remarkable difference was 

 observed in the lines observed at sun-spot 

 maximum and minimum. This continued 

 till about 1895, another ten years. As the 

 curve of iron lines went up, the curve of 

 'unknown' lines came down; there were 

 therefore crossings of the curves which 

 might, on the hypothesis before referred 

 to, be taken as the times at which the tem- 

 perature of the solar system had a mean 

 value. These crossings turned out to be 

 about half-way between the maxima and 

 minima of the spotted area which had to be 

 considered as the times at which the sun 

 was hotter and colder than the mean. 



We were then brought into the presence 

 of three well-marked stages of solar tem- 

 perature—it was no longer a question 

 merely of spots and no spots, but of heat 

 pulses. 



The next point was to study these heat 

 ptdses in relation to the Indian rainfall, 

 and it was found that in many parts of 

 India the plus and minus heat pulses on 

 the sun, which, of course, occurred imme- 

 diately after the time of mean temperature, 

 when the sun was getting either hotter or 

 colder, were accompanied by pulses of rain 

 in the Indian Ocean and the .surrounding 

 land. It was next found, from a study of 

 the Indian Famine Committee's reports, 



