'30 REPOKT — 1875. 



wliile large capacities have tlie effect of lessening sucli fluctuations. A sandy soil, 

 Bucli as tfiat of the great African desert, although capable of exhibiting a very high 

 temperatui-e diu-ing the day, becomes cooled down during the night, and is one of ' 

 . ^the worst substances for storing up the heat derived from sunshine. Water, on the 

 contrary, stores up such heat better than almost any other bodj'i 



The grounds for the author's conclusions as to the thermal properties of 

 water wer§ fully exhibited in a paper published in the 'Atlantis ' and the ' Philoso- 

 phical Magazine' for 1859, and subsequently republished wholly or partially in 

 some foreign scientific journals. Instead of the dictum expressed by Sir John 

 Herschel, we may make the following almost diametrically opposite announcement. 

 The effect of land receiving simshine is to throw off the heat it receives, not only 

 into the atmosphere, but into the interplanetary spaces by night as well as by day ; 

 and thus, although it rapidly produces a considerable increase of temperature in the 

 stratum of an- immediatelj^ in contact with it during the day, it is iU adapted for 

 storing up and retaining the thermal energy it has received. Water is much more 

 effective in this respect ; heat penetrates to a gi-eater depth within it, and afterwards 

 becomes more steadily absorbed, owing to the much higher specific heat of water 

 and the ratchet-wheel action exercised on the luminous heat-rays from the sim, 

 which, on their transformation into obscm-e rays, cannot again return through the 

 liquid. Prof. Tyndall has established that this ratchet-wheel action of water exists 

 for water in its vaporous state as well as in its ordinary condition, and accordingly 

 that a vapour-charged atmosphere, though comparatively diathermanous for the 

 sun's heat-rays, which pass without considerable loss to the earth's surface, j'et this 

 moist air almost completely stops the radiation of non-luminous heat from the 

 ground. 



In his earlier researches the author pointed out how the distribution of heat over 

 the Caribbean Sea and the Gulf of Mexico, and the general laws followed in the distri- 

 bution of the isothermal lines of islands, and especiallj' in the British Isles, confirmed 

 these views. 



At this Meeting a remarkable confirmation is afforded by the results communi- 

 . cated by Captain Toj-nbee, whereby he has shown that in the equatorial Atlantic 

 the water-surface temperature is higher than the air-temperature above it. With 

 regard to geological climate, these conclusions were long since adopted and acknow- 

 ledged by Professor Phillips. They have been also adopted, but not acknowledged, 

 bj' other writers besides the distinguished and lamented geologist to whom this 

 Association is so gTcatly indebted. 



The distribution of terrestrial temperature depends essentially both on oceanic 

 and atmospheric cm-rents, and the question of oceanic circulation is thus closely 

 connected with that of climate. Much attention has been recently excited by two 

 theories — one which ascribes oceanic currents to the motions of the atmosphere, and 

 the other to the direct influence of gravity on masses of water unequally heated. 

 This or the gravitation-theory has been already broached in the author's paper 

 already referred to. It has been recently pm'sued in more detail, and with his 

 usual ability, by Dr. Carpenter. 



On the possible Tn-fluence on Climate of tJie suhstitution of Water for Land in 

 Central and Northern Africa. By Prof. H. Hennesst, F.R.S. 



The author referred to the fact that more than six years since he had put forward 

 proofs of the connexion between some of the hot winds which blow from the south- 

 west in Central and Southern Europe with the currents of the Atlantic and not with 

 the desert of Sahara, as has been usually supposed *. Similar views had been also 

 enunciated by Prof. 'W^ild, Director of the Physical Observatory of Russia, Prof. 

 Dufour of Lausanne, Dr. Hann, and other meteorologists. The attention excited 

 by the great midday heat of Central Africa caused many to overlook the 

 remarkably low nocturnal temperature, and thus to ascribe to the desert a thermal 

 influence which it does not possess. The conclusions which the author has established 

 with regard to the physical properties of water in connexion with climate (of which 



* Proceedings of the Eoyal Irish Academy, vol. x. p. 41)6. 



