360 



Mr. J. P. Harrison on the Relation of [Feb. 14, 



Table V. 



Monthly jMeans of Vapour-Tension at Greenwich at 22^ 0^ and 2^ 

 (1842-47). 



Hour. 



March. 



April. 



May. 



June. 



July. 



August*. 



Sept. 



h 

 22 



•236 



0-284 



0-357 



0^437 



0-462 



0-473 



0-427 







•246 



0-294 



0-365 



0-446 



0-469 



0-483 



0-441 



2 



+ •249 



+0-297 



+ 0-368 



+0-449 



+0-475 



+ 0-486 



+ 0-444 



The mean increase of yapour from lO'* to noon is '010; and the mean 

 increase from noon to 2*^ is '0034. In six of the months the mean increase 

 is in each case '003 f. 



The results accord with the assumed date of maximum radiation. 



Actinometer-ohservations. — The observations of the actinometer made by- 

 Mr. Nash at the Royal Observatory in 1864 supply further evidence of the 

 fact that the maxima of radiation occur in the autumn, and after 0^. 



I have extracted the mean results of the several groups of observations 

 which were taken at or near the same elevation of the sun, in the months 

 of August and September in autumn, and in the months of March and 

 April in spring, and find that the value of the mean increase of the scale- 

 readings in the autumn is about 100 per cent, greater than in the 

 springs. 



The mean increase of the scale-readings in March and April is 19*4. 

 The mean increase of the scale-readings in August and September is 39*6. 

 The difference is 20*2, at the same mean altitude of the sun. 

 The means of the contemporaneous observations of the vacuum-thermo- 

 meter on the grass were as follows : — 



In March and April 70°-3 F. 



In August and September . . . 84°* 7 F. 



The difference is 14°- 4. 



And the mean tension of vapour for the four months is, in March and 

 April '23, and in August and September '34. 



* August is the only month in which the mean tension of vapour is higher at p.m. 

 than at vm. 



t With the exception of July (when there is the minimum difference of -007 between 

 the results at 10^ a.m. and noon) and of September (when there is the maximum difference 

 •014), the mean increase in tension at noon in the several months is also remarkably 

 regular. 



% See ' Greenwich Meteorological Eesults,' 1864. p. xxxviii. 



