152 



NATURE 



\yune 16, 1 88 1 



This has been brought prominently before us of late years by Dr. 

 Hunter, Director-Genera) of Statistics in India, who has shown 

 tliat famines are most frequ-'nt at Madras about the years of 

 minimum sun-spots — years which were likewise associated with a 

 dimini.^hed rainfall - 



In summing up the rainfall evidence we ought to bear in mind 

 that the direction as ^ell as the iatensity of the earth's convec- 

 tion currents is no doubt altered by solar variability. And if 

 we at the same time reflect how very local rainfall is, we cannot 

 expect that the same rule regarding it should hold for all the 

 varijus stations of the earth's surface. But on the whole there 

 appears to me to be evidence that we have most rainfall during 

 most sun-spats. Of course we know little or nothing of varia- 

 tions in the rainfall at sea. 



I have already mentioned that the magnetic storms of the 

 earth are most frequent during yeai's of maximum sun-spots, 

 and tlie very same thing may be said of wind-storms. Dr. 

 Meldrum has fovmd that there are more cyclones in the Indian 

 Ocean in years when there are most sun-spots, and fewest 

 cyclones in ye.ars when there are fewest sun-spots. M. Poey 

 has proved a similar coincidence between the hurricanes of the 

 West Indies and the years of maximum sun-spots, and I 

 believe that a similar conclusijn has been arrived at with 

 regard to the typhoons of the Chinese seas. 



In 1877 Mr. Henry Jeula of Lloyds and Dr. Hunter found 

 that the percentage of casualties on the registered vessels of the 

 United Kingdom was 17?, per cent, greater during the maximum 

 two years than duruig the minimum two years in the common 

 sun-spot cycle. 



We may therefore imagine that the wind as well as the rain of 

 the earth is most violent during years of maximum sun-spots. 



We come now to tlie pressure of the air. If there were no 

 sun the pressure of the air would ultimately distribute itself 

 equally where it is now unequal. This inequality is no doubt 

 caused by the sun, and we sliould expect it to be most pro- 

 nounced when the sun has most power. It is also different in 

 summer and winter. In summer we generally find a low bar j- 

 meter in the centres of great continents, and a high barometer 

 over the sea ; while during winter we have the converse of this, 

 or a liigh barometer over continents and a low barometer at sea. 

 I think it likely th.at the true relation between the v.ariations of 

 sun-spots and of barometric pressure will ultimately be dis- 

 covered by means of the admirable weatlier-mips of the United 

 States ; meanwliile, however, especially in India, something has 

 already been done in this direction. 



If we regard the distri'mtion cif isobaric lines, that is to say of 

 lines of equal barometric pressure, we shall find that the Indo- 

 Malayan region is one which for the mean of the year has a 

 barometric pressm-e probably below the average. Now during 

 years of powerful solar action we might imagine that this 

 peculiarity would be increased. But this is precisely what all 

 the Indian observers have found for years wi h most sun-spots. 



On the other hand Western Siberia in the winter season has 

 a pressure decidedly above the average, and we should therefore 

 imagine that during years of powerful solar action the winter 

 pressure would be partii:ularly high. This .again is the state of 

 tilings that Mr. Blanford has found in his discussion of the 

 Russian stations to correspo.id with years of mott sun-spots. 



It therefore appears to me that the barometric evidence as far 

 as it goes is favourable to the belief that years of maximmn sun- 

 spots are years of greatest solar power. 



I come now to consider the question of temperature. Mr. 

 Baxendell was the first to conclude that the distribution of 

 temperature under different winds, like that of barometric pres- 

 sure, is very sensibly influenced by the changes which take place 

 in solar activity. In 1870 Prof. Piazzi Smyth published the 

 reults of observations made from 1S37 to 1S69 with thermo- 

 meters sunk in the roc'.< at the Royal Ooservatory, Edinburgh. 

 He concluded from these that a heat wave occurs about every 

 eleven years, its maximum slightly lagging behind the minimum 

 of the sun-spot cycle. In 1871 Mr. E.J. Stone examined the 

 temperature observations recorded during thirty years at the 

 Cape of Good Hope, and came to the c -inclusion that the same 

 cause which leads to an access of mean annual temperature at 

 the Cape leads equally to a dissipation of sun-spots. Dr. W. 

 Kbppen in 187J discussed at great length the connexion be- 

 tween sun-spots and terrestrial temperature, and found that in 

 the tropics the maximum temperature occurs fully a year before 

 the year of minimum sun-spots : while in the zones beyond the 

 tropics it occurs two year^ after the minimum. The regularity 



and magnitude of the temperature wave is most strongly marked 

 in the tropics. 



The temperature evidence now given appears at first sight to 

 be antagonistic to that derived from the other elements, both of 

 magnetism and meteorology, and to lead us to conclude that the 

 sun heats us most when there are fewest spots on its surface. 

 This conclusion will not, however, be strengiheiiel if we discuss 

 the subject with greater minuteness. Scientifically, we may 

 regard the earth as an engine, of which the sun is the furnace, 

 the equatorial regions the boiler, and the polar regions the con- 

 denser. Now this engine works in the following manner. Hot 

 air and vapour are carried along the upper regions of the atmo- 

 sphere from the equator to the poles by means of the anti-trade 

 winds, while in return the cold polar air is carried along the 

 surface of the earth from the poles to the equator, forming what 

 is known as the trade winds. Now whenever the sun's heat is 

 most powerful, both trades and anti-trades should, I imagine, 

 be most powerful likewise. But we live in the trades rather 

 than in the anti-trades — in the surface currents, and not in the 

 upper currents of the earth's atmosphere. When the sun is 

 most powerful, therefore, is it not possible that we might have a 

 particularly strong and cold polar current blowing about us? 

 The fame thing would happen in the case of a furnace-fire — the 

 stronger the fire the more powerful the hot draught up the 

 chimney — the more powerful also the cold draught from without 

 along the floor of the room. It might thus f illow that a man 

 standing in the furnace room near the door might be chilled rather 

 than heated when the furnace itself was roaring loudest. In 

 fact temperature is a phenomenon due to many causes. Thus 

 a low temperature may be due 



(i) To a deficiency in solar power. 



(2) To a clouded sky. 



(3) To c«old rain. 



(4) To cold winds. 



(5) To cold water and ice. 



(6) To cold produced by evaporation. 



(7) To cold produced by r.adiation. 



Now Mr. Blanford, the Indian ob erver, has recently shown 

 that a low temperature of the air and soil is accompanied in the 

 stations which he has examined by a copious rainfall and by a 

 large number of clouds. If therefore we regard a high rainfall 

 as the concomitant of many sun-spots, we must not be surprised 

 if this is sometimes accompanied with a low tem;ieratnre, nor 

 hastily conclude from this lowering of temperature that the sun 

 is less rather than more powerful. Considerations of this nature 

 have induced me to think that the true connexion between sun- 

 spots and terrestrial temperature is more likely to be discovered 

 by a study of short-period inequalities of sun-spots than by that 

 of the eleven-year period in which there is time enough to change 

 the whole convection system of the earth. I have accordingly 

 discussed at some length two prominent sun-spot inequalities of 

 short periods (about twenty-four days), and endeavoured to see 

 in what way they affect the terrestrial temperature. From this 

 it appears that a rapid increase of sun-spots is followed in a 

 day or two by an increase of the diurnal temperature range at 

 Toronto. No v an increase of diurnal temperature range surely 

 denotes an increase of solar energy, an ' we are thus led to 

 associate an increase of solar heat with a large development of 

 spots. 



I have thus brought before you a quantity of evidence, chiefly 

 indirect, tending to prove that the sun's rays are most powerful 

 when there are most spots. But you will naturally adi why I 

 have not given you any direct evidence on this point. Is it not 

 possible, you ask, to measure the direct heating effect of the 

 sun's ray,, so as to decide the question without fuither circum- 

 locution ? Now, strange to say, this has not been done. 



We call an instrument that measures the sun's direct influence 

 an actinometer, and I will now briefly allude to two such in- 

 struments, one for measuring the chemical effect of the sun's 

 raj'S devised by Dr. Roscoe, and another for measuring the 

 heating effect of the sun's rays, devised by myself. (The lecturer 

 here described the mode of actim of these actinometers.) 



But the use of such instruments is rather a problem of the 

 future than of the past. Hitherto it cannot be said that we have 

 determined by actual o'jservation whether the sun's rays are 

 more powerful or less powerful at times of maximum sun-spots. I 

 may, however, quote the actinometrical observations made in 

 India at Mussooree and Dehra by Mr. J. B. N. Hennes-ey as 

 confirming, so far as the evidence goes, the hypothesis of 

 gi-eater solar energy at maximum than at minimum epochs. 



