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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



turbances on the lines were not of so marked 

 a character. He holds as an established fact 

 that "earth-currents, disturbed magnetometers, 

 and aurorae, are parts of the same phenomenon," 

 and, in a recent letter to one of the writers of 

 this article, he reaffirms his conviction regard- 

 ing the relationship between earth-currents, tele- 

 graphic disturbances, and sun-spots. 



The second class of phenomena, in which a 

 periodicity coinciding with the sun-spot cycle is 

 believed to have been discovered, has reference 

 to solar radiation and thermometric variations. 

 For reasons which would require too much space 

 to detail, various difficulties complicate this line 

 of research, and we should state, at the outset, 

 that the evidence is less complete and satisfactory 

 than that which connects magnetic disturbances 

 and rainfall with sun-spots. A moment's consid- 

 eration will show the kind of complication to which 

 we refer. If the earth had no atmosphere, all the 

 solar energy would be incident and operative on 

 the earth's surface, where perforce our measuring 

 instruments are placed. But the earth has an 

 atmosphere, which is the vast scene of the play 

 of the solar energies ; and the work done there 

 is of such a nature that the more energy there is 

 in operation, the more effectively is the direct 

 energy of the sun screened from the surface. 

 Further, there is not wanting evidence to show 

 that the vapor of water, like the vapors of the 

 metals, exists in various molecular conditions, 

 some of which are transparent and others opaque 

 to those rays which affect our thermometers. 1 

 The thermometric inquiry divides itself into sev- 

 eral distinct branches, such as the direct solar 

 radiation or calorific intensity of the sun's light, 

 the daily temperature range, and the mean an- 

 nual temperature. We shall very briefly state 

 the conclusions at which observers have arrived 

 during the last ten years, without criticism or 

 any expression of opinion. 



In 1867 Mr. Joseph Baxendell communicated 

 the results of a scrutiny of the Solar Radiation 

 Registers, kept at the Radcliffe Observatory, Ox- 

 ford, from 1856 to 1864. He came to the fol- 

 lowing conclusions, among others : 1. That the 

 calorific intensity of the sun's light is subject 

 to periodical changes, the maxima and minima 

 of which correspond respectively with those of 

 sun-spot frequency. 2. That it seems probable 

 that the heating rays of the sun consist of two 



1 There is evidence to suggest that the aqueous va- 

 por prod need at the period of minimum sun-spots 

 would be more transparent to the heat-rays than that 

 produced at other times.— J. N. L. 



kinds, differing in intensity, and subject to pe- 

 riodical changes ; the times of maxima of one 

 kind, and those of minima of the other, corre- 

 sponding respectively to the times of maximum 

 frequency of solar spots. Mr. Baxendell also 

 pointed out a connection between the mean 

 monthly variation of solar radiation on cloudless 

 days and the mean monthly daily range of the 

 magnetometer. In 1871 he published his further 

 researches on the changes in the distribution of 

 barometric pressure, temperaturej and rainfall, 

 under different winds, during a period of solar- 

 spot frequency. He found that changes had 

 taken place in the three elements under discus- 

 sion, which corresponded very closely in the 

 times of their maxima and minima with those of 

 sun-spot frequency. In 1S75 Mr. H. T. Blandford, 

 Meteorological Reporter at Calcutta, stated, from 

 experiments conducted in Bengal : " The result is 

 to me very striking, and, if not absolutely con- 

 clusive as to the direct variation of the sun's heat 

 with the number of spots and prominences, cer- 

 tainly, as far as it goes, strongly confirms Mr. 

 Baxendell's conclusions." ' In the same year 

 Professors Balfour Stewart and Roscoe, from an 

 investigation of the heating effects of the sun, 

 came to the conclusion that there is more sun- 

 shine at London in years of maximum than in 

 years of minimum solar disturbance. Next year, 

 1876, Prof. Balfour Stewart found that the win- 

 ter temperature range at Kew apparently de- 

 pends on the sun-spot period, being greatest at 

 times of maximum sun-spots, and least at times 

 of minimum sun-spots. This year, 1877, he has 

 raised, and produced evidence upon, the interest- 

 ing question whether the mean daily range does 

 not depend, among other influences, on the state 

 of the sun's surface with regard to spots. 



Meanwhile, another series of observations had 

 been going on, not with black-bulb thermometers 

 fdr solar radiation, but with reference to the mean 

 annual temperature. In 1870 Prof. Piazzi Smyth, 

 the Astronomer Royal for Scotland, published 

 the result of observations made from 1837 to 

 1869, with thermometers sunk in the rock at the 

 Royal Observatory, Edinburgh. He came to the 

 conclusion that a great heat-wave occurs every 

 eleven years and a fraction, its maximum slightly j 

 lagging behind the minimum of the sun-spot cy- 

 cle. Next year, 1871, Mr. E. J. Stone, the As- 

 tronomer Royal at the Cape of Good Hope, ex- 



1 We should add, however, that a communication 

 has just appeared {Nature, October 11, 1877), from Mr. 

 Hill, in Northern India, differing from Mr.Blandfcud's j 

 conclusions. 



