3Koi)aI institution of ffireat iiStft^.^®^ 



WEEKLY EVENING MEETINd'^ 



Friday, January 21, 1887. 



William Huggins, Esq. D.C.L. LL.D. F.R.S. Manager^ 

 President, in the Chair. 



Sir William Thomson, LL.D. F.R.S. M.B.L 



PROFESSOR OF NATURAL PHILOSOPHY IN THE DNIVERSITT OF GLASGOW. 



The Sun's Heat. 



From human history we know that for several thousand years the 

 sun has been giving heat and light to the earth as at present, possibly 

 with some considerable fluctuations, and possibly with some not very 

 small progressive variation. The records of agriculture, and the 

 natural history of plants and animals within the time of human 

 history, abound with evidence that there has been no exceedingly 

 great change in the intensity of the sun's heat and light within the 

 last three thousand years ; but for all that, there may have been 

 variations of quite as much as 5 or lO^per cent., as we may judge by 

 considering that the intensity of the "solar radiation to the earth is 

 6 J per cent, greater in January than in July ; and neither at the 

 equator nor in the northern or southern hemispheres has this differ- 

 ence been discovered by experience or general observation of any 

 kind. But as for the mere age of the sun, irrespective of the question 

 of uniformity, we have proof of something vastly more than three 

 thousand years in geological history, with its irrefragable evidence of 

 continuity of life on the earth in time past for tens of thousands, and 

 probably for millions of years. 



Here, then, we have a splendid subject for contemplation and 

 research in Natural Philosophy or Physics — the science of dead 

 matter. The sun, a mere piece of matter of the moderate dimensions 

 which we know it to have, bounded all round by cold ether,* has been 

 doing work at the rate of four hundred and seventy-six thousand 

 million million million horse-power for three thousand years ; and at 



* The sun warms and lights the earth by wave motion, excited in virtue of 

 his white-Lot temperature, and transmitted through a material commonly called 

 the luminiferous ether, which fills all space as far as the remotest star, and has 

 the property of transmitting radiant heat (or light) without itself becoming 

 heated. I feel that I have a right to drop the adjective luminiferous, because 

 the medium, far above the earth's surface, through which we receive sun-heat 

 (or light), and through which the planets move, was called ether 2000 years 

 before chemists usurped the name for " sulphuric ether," " muriatic ether," and 

 other compounds, fancifully supposed to be peculiarly ethereal ; and I trust that 

 chemists of the present day will not be angry with me if I use the word ether, 

 pure and simple, to denote the medium whose undulatory motions constitute 

 radiant heat (or light). 



YoL. XII. (No. 81.)^ B 



