TRANSACTIONS OF THE SECTIONS. 39 



On the Decrease of Ti'mpendure with Height on the Earth's Surface. 

 By Trofossor Hunnessy, F.ILS. 



If the air were perfectly still, the temperature at any point in the atmosphere 

 would depend on its density, the heat absorbed from the solar rays, the heat ob- 

 tained by convection from the earth, and the losses of lieat by radiation. 



Of these the first has been almost exclusively considered. This is especially so 

 ill all investigations for the ascertainment of lieights by the barometer. The exclu- 

 sion of the other causes of variation of temperature with heig-lit may be admissible 

 in considering the condition of a vertical column of air resting on a horizontal plane ; 

 but the problem assumes a very different character when tlie decrease of tempera- 

 ture witli height along a very gradually sloping surface is considered. Such a 

 surface is constantly communicating its temperature by convection currents to the 

 overlying air, and the temperature of this air will depend on the extent, form, and 

 physical properties of the underlying surface. If we suppose a flat plain on the 

 level of the sea, an observer in a balloon at a height of 1000 feet would find the 

 temperature almost unaffected by convection and dependent upon density. If, now, 

 a steep mountain is superimposed on the plain and reaching to the observer, the 

 conditions become altered. If a mountain of a gradual slope be superimposed, the 

 alteration will be still gi-eater ; and if the entire plain were elevated up to 1000 

 feet so as to form an extensive tableland, the change of conditions would be very 

 remarkable. 



It follows that the law of vaiiation of temperature with height above the level 

 of the sea cannot be considered as uniform. The decrease is most rapid in going 

 up through a vertical column of air, as in balloon ascents. It is slower along 

 mountain sides, and slowest along gradually sloping plains or tablelands. 



From an examination of tlie records of many observations, it appears that the 

 decrease of temperature in balloon ascents is nearly one degree Fahrenheit for 300 

 feet, while for tablelands it is so slow as from 500 to 800 feet for one degree. 



The author referred to a number of observations made in different countries con- 

 firming the general conclusions to which he has been led. 



On the Distribution of Temperature over the British Islands. 

 By Professor Hennesst, F.B.S. 



The author referred to his former researches on the distribution of temper.^ture 

 over islands surroimded by heat-bearing cun-euts and his demonstration that many 

 of the isothermal lines in such islands must necessarily be closed curves*. He had 

 originally illustrated his conclusions by the results of observations taken in the 

 British Islands, and the isothermal lines laid down from such observations were 

 found to be in perfect harmony with the law he had proved. In order to render 

 this manifest he tabulated together the temperature of each, stating its latitude, 

 longitude, height above the sea, and horizontal distance from the nearest sea coast. 

 The actual temperature of any place is affected by all of these elements. In laying 

 down the isothermal lines the actual temperatures unaltered by any so-called cor- 

 rection for height were always employed. The stations were arranged according to 

 temperature, and thus isothermal groups were immediately discovered. If the more 

 recent collection of temperature results for the British Isles compiled by Mr. Buchan 

 in the Journal of the Scottish Meteorological Society be treated in this way and the 

 arbitrary and erroneous addition of 1° per every 300 feet in height be omitted, his 

 results will conform to the law enimciated by the author. 



* See Brit. Assoc. Eep. 1857, pt. 2, p. 30; Atlant^is, i. 185S, pp. 39G-413; Phil. Mag. 

 xvi. 1858, p. 241 ; Eoyal Society Proc. ix. p. 324 ; " On the Laws which regulate the 

 Distribution of Isothermal Lines," Atlantis, ii. p. 201 ; American Journal of Science, xxvii. 

 p. 328. Copies of the temperature-maps are also partly reproduced in Eeport of Horti- 

 cultural Congress at London in 186G ; Journal of the R. Dublin Society, vol. for 1870-71 

 Eeport of the Commission on Ojstcr Fisheries, 1871. 



