202 MB. T. HOPKINS ON THE 



of the mountain is higher by 2,000 feet. The higher parts 

 stop the passage of the wind, which makes its way where 

 there is the least resistance, and this is over the pass of the 

 mountain ridge ; and as the horizontal rush of air is here 

 particularly strong, any rain that is there formed, or that 

 has been carried thither, will be liable to be borne forward 

 until the air loses some of its velocity in the comparatively 

 open space beyond the Pass where the rain is likely to be 

 deposited — just as nmning water deposits sand when it 

 reaches a wider and comparatively still part of a river. 



In the case stated, the air was supposed to be saturated 

 with vapour, but if it should not be fully saturated, but 

 have a dew-point of, say 1° below the temperature, the only 

 difference would be that condensation would not begin until 

 the mass of air climbed 300 feet. When the dew-point is 

 two degrees below the temperature, the air must ascend 

 600 feet before condensation begins ; and the more the 

 dew-point is below the temperature the higher must the air 

 ascend before condensation will commence; but when it 

 does begin, the process will be of the nature that has been 

 described when the temperature and the dew-point were 

 the same. And it should be recollected that it is not alone 

 the vapour that is near the surface of the earth that may be 

 condensed in the way described, but the whole vertical 

 column may be affected in the same way as that part which 

 is near to the surface. The vapour that is 300 or 600 feet 

 distant from the surface of the low ground, may be equally 

 raised and condensed with that which rests on the surface, 

 seeing that the whole vertical column of the atmosphere 

 may be raised by the obstruction presented by the moun- 

 tain to the passage of the wind. And the rain that is formed 

 in the higher, as well as that in the lower regions, is during 

 its descent always liable to be carried forward by the wind. 

 We see, then, why the largest quantity of rain should fall 

 in Seathwaite when a south-west wind blows from the sea 



