216 



Mr. S. A. Hill. On the Constituent of [Dec. 22, 



VII. " On the Constituent of the Atmosphere which absorbs Ra- 

 diant Heat." By S. A. Hill, B.Sc, Meteorological Reporter 

 for the North-Western Provinces and Oudh, India." Com- 

 municated by Lieut-General R. Strachey, R.E., F.R.S. 

 Received December 14, 1881. 



Notwithstanding the ingenuity with which Dr. Tyndall has made 

 nse of the most recent physical appliances to support and confirm the 

 results of his classical researches concerning the behaviour of gases and 

 vapours with regard to radiant heat, his conclusions, in so far as they 

 relate to the comparative diathermancy of dry air and water vapour, 

 have not yet met with general acceptance among meteorologists. 

 There is even, on the part of some, an evident reluctance to accept 

 the decision of laboratory experiments on the question of atmospheric 

 absorption as final, however ingenious, varied, and consistent with 

 one another the experiments may be. 



I have, therefore, attempted to approach the question from another 

 side, and to determine, if possible, what constituent of the atmosphere 

 has the greatest absorptive power, by means of the actinometric observa- 

 tions so carefully made at Mussooree and Dehra by Messrs. J. B. N. Hen- 

 nessey, F.R.S. , and W. H. Cole, M.A., in conjunction with the records 

 of meteorological observations made at the same or neighbouring- 

 places in the same months of the year. The actinometric observations 

 were made between the 27th October and 4th November, 1869, and 

 between the 31st October and 19th November, 1879. Abstracts of the 

 results have been published by Mr. Hennessey in the " Proceedings," 

 vol. xix, p. 229, and vol. xxxi, p. 154. In both years the observations 

 were made with two actinometers of the Rev. Gt. C. Hodgkinson's 

 form, marked A and B, each of which appears to have remained 

 absolutely unaltered during the ten years. The results taken from 

 Mr. Hennessey's tables are those expressed in units equal to a tenth of 

 a millimetre of the scale of the instrument A, glass off. 



The observations which serve to throw most light on the question 

 under discussion are those of the long diurnal series, from 8 a.m. to 

 4 p.m., made on the 4th November, 1869, and the 12th and 14th 

 November, 1879. In Tables I, II, III, the observed values of the 

 radiation received in three minutes at each hour* are compared with 

 certain symmetrical values computed in the following way. 



It has been assumed that Jamin and Masson's law of absorption 

 holds good for each day at both stations ; that is to say, that the 

 logarithm of the heat received varies inversely as the thickness of the 

 atmosphere traversed, or, in other words, that the quantity of heat 



* On the 4th November, 1869, the mean time of each set of observations was 

 several minutes after the exact hour. The values given in Table I have been in- 

 terpolated from Mr. Hennessey's figures by means of parabolic formula extending 

 over three hours at a time. 



