Hennessy — On Temperature over Great Britain, 8^r. 709 



XLIV. — On the DrsTRrBrTiojf of Tehpeeatuee ovee Geeat Eeitaen" 

 AKD Ieelaa^d. By He^tet IIe:n"i^'ESST, P.R.S., Professor of Applied 

 Mathematics in the Koyal College of Science for Ireland. (With a 

 Map.) 



[Bead, June 28, 1886.] 



In islands surrounded by waters whose temperature is in excess of the 

 temperature of the air, I have long since shown that the distribution 

 of temperature must be represented by isothermal lines, similar in out- 

 line to the coasts of the islands, and with the centres of the isothermals 

 transported in the direction of the nearest pole of the earth. ^ This 

 result has never been controverted, but it has been confirmed so com- 

 pletely by observations as to become an established fact of climatology. 

 But as the distribution of temperature may be represented either from 

 the actual facts collected or by the reduction of the temperatures to 

 their supposed values at the sea level, it may be useful to make some 

 remarks on the variation of temperature with height in the atmosphere, 

 and on the utility of the so-called corrected temperatures obtained by 

 reduction to the sea level. 



If the air were perfectly still, the temperature of any molecule 

 would depend — (1) upon the density of the strata in which it is; 

 (2) upon the heat which it absorbs from the solar rays by which it is 

 traversed; (3) upon that which it absorbs from terrestrial radiation, 

 and upon what it loses by radiation into space. The most important 

 of these, (1), has been usually taken into consideration by physical 

 writers, especially in treatises of the question of ascertainment of 

 heights by the barometer. But the air is never perfectly still ; even' 

 in a calm day, frequent and rapid currents promote interchanges of 

 temperature between its several strata. Close to the earth's surface 

 the energy and numbers of such small air currents become so great as 

 to cause a rapid variation of temperature ; and it follows that the 

 temperature of a molecule of air is thus a function of its distance 

 from the nearest surface capable of imparting heat to it, or of absorbing 

 heat from it. It is also a function of the extent, form, and physical 

 character of such a suiiace. To illustrate these points, let us suppose 

 the line C D to represent a flat plain nearly at the level of the sea. 

 An observer stationed in a balloon at B, 1000 feet above the plain, 

 would find the temperature of the air cooled by contact with its sur- 

 face. If a mountain with a much broader base be superimposed, but 



' The late Dr. Lloyd, Provost, T. CD., long since acknowledged tliis result. "The 

 actual isothermal lines of Ireland are inflected in passing from the sea to the land, 

 and must even, in part of the island — as Mr. Hennessy has pointed out — take the 

 form of closed carves, dependent on the position of the places traversed T\'ith respect 

 to the coast-line." — Lecture on "The Climate of Ireland and Currents of the 

 Atlantic" : Dublin, 1865. See Atlantis, vol. i., p. 396 ; Philosophical Magazine, 

 October, 1858 ; and Proceedings of the Eoyal Society, vol. ix., p. 324. 



R.I. A. PJIOC, SEK. II., VOL. IV. — SCIEXCE. 3N 



