1889.] and May 1888 in the Doab and Rohimand. 161 



weather this does not take place until a height but seldom attained is 

 reached. In the cases now under discussion it is probable that, owing 

 to the moist air brought in by easterly winds and the upward diffusion 

 of the vapour during the three or four very hot days preceding the 30th 

 April, the plane at which condensation commenced when the rapid fall 

 of temperature set in that afternoon was unusually low for the time of 

 year. If we adopt Hann's formula for the vertical distribution of 

 aqueous vapour, and compute its constants from the observations of 

 Bareilly, Roorkee, Dehra, Mussooree, Chakrata, Ranikhet, and Pithora- 

 garh (a large number of observations of this somewhat erratic element 

 being desirable), we get the following results for 4 p. m. each day : — 



29th April, log 7352 - -0316 h. 



80th „ log f - 9-7412 - -0275 h. 



1st May, log f = 9'7630 - -0370 h. 

 In these formula) / is supposed to be expressed in decimals of an 

 inch of mercury and h in thousands of feet. In Hann's general formula 

 for all seasons and places the co-efficient of h is -0462. On the 30th 

 of April thei'efore the rate of diminution of vapour pressure with 

 increase of height was only -Hf or -| of the average rate, whilst the 

 decrease of temperature from 5,000 feet upwards was very rapid. On 

 the 1st May, the vapour diminished rather more quickly, but the decrease 

 of temperature was so rapid as to overtake it and cause condensation at 

 the same or a lower level. If wo compare the pressure of satm-ated va- 

 pour at the temperature of the air with the actual vapour tension given by 

 the abovs formulas, we find that, on the 29th, the relative humidity was 

 below 50 per cent., even at 10,000 feet, whilst on the 30th condensation 

 commenced at about 9,300 feet and on the 1st May at 8,000 feet : — 



Tension of vapour at 4 P. m. 



neight. 



29th April. 



30th April. 



1st May. 



Satiiratocl. 



Actual. 



Satarated. 



Actual. 



Saturated. 



Actual. 



Sea level 

 1,000 feet 

 2,000 „ 

 3,000 „ 

 4,000 „ 

 5,000 „ 

 6,000 „ 

 7,000 „ 

 8,000 „ 

 9,000 „ 

 10,000 „ 



2-307" 

 1-809 

 1-481 

 1-221 

 1-029 

 •881 

 •773 

 •693 

 •636 

 •600 

 •577 



-514" 



•502 



-463 



•428 



-395 



•365 



•337 



•311 



•287 



-205 



-245 



1-645" 

 1-537 

 1-404 

 1-253 

 1-091 

 •923 

 •760 

 •606 

 •468 

 ■350 

 ■251 



■551" 

 ■518 

 ■488 

 ■459 

 •432 

 •406 

 ■383 

 ■360 

 ■338 

 ■318 

 (-300) 



1-127" 

 1-105 

 1043 

 ■945 

 •821 

 •684 

 -514 

 -411 

 -295 

 •199 

 -122 



•585" 

 •538 

 •494 

 •453 

 -416 

 •382 

 -351 

 322 

 ( 296) 

 (■272) 

 (-250) 



