METEOROLOGY AND ALLIED SUBJECTS. 



283 



position and physical cause of the existence of a zone of maximum rain- 

 fall in the Northwestern Himalayas. 



The existence of such a zone was already known to General Strachey 

 in 1849, but with the help of a large number of observations Hill is able 

 to show that the relative quantity of rain (R) falling at the high and 

 low stations is very closely represented by the formula 



" E = l + 1.92h-0.40i,2 + 0.02 1,3 



in which fe is the relative altitude of the upper stations above the plain. 

 Hie differential of this formula gives the equation for the determination 

 of the value of li corresponding to the maximum relative rainfall, which 

 position is easily found to be )i = 3,1G0 feet relative to the lower stations 

 or 4,100 feet above the sea-level. As regards the cause of this excessive 

 rainfall at a definite and moderate altitude, Hill adopts the explanation 

 suggested by General Strachey. The variation in the tension of vapor 

 (p)j up to an altitude of 1,200 feet, is closely represented by the formula 



in which a and /9 are certain constants that must be determined from 

 observations. 



Dr. Hann has shown that the tension of vapor can be closely repre- 

 sented \>j the barometric formula 



logp = logpo-^- 



where the constant c has about a value of 6,500. According to each of 

 the formulae the measure of the diminution of tension of vapor is greater 

 in proportion as the altitude is less. If once the temperature sinks to 



the dew-point the quotient -,^ is a measure of the quantity of the pre- 



cipitation, and we must therefore expect that the rain is heaviest in that 

 zone where on the average a mass of air ascending from the lower 

 plains reaches the point of saturation with aqueous vapor. This zone 

 can be determined with sufl&cient accuracy for the region studied by 

 Mr. Hill, if we seek the altitude at which the temperature during the 

 rainy season is equal to that of the dew-point at Eoorkee. A computa- 

 tion of this kind is made by Mr. Hill for three months, with the following- 

 results: 



Month. 



Altitude. 



Jnly 



Aujust ... 

 September 



Feet. 

 :!, 141 

 2,710 

 3,099 



The mean altitude at which the rainfall may be expected to be great, 

 est lies, according to this theory, about 3,000 feet above the lower i)lain- 



