432 JOURNAL OF THE PLYMOUTH INSTITUTION. 
every 10 cubic feet of the atmosphere, and in this room the 
quantity would weigh more than 3 lbs., and measure more than 
1} quart. Hence, when we take into consideration the millions 
of cubic miles the air fills, we can understand where the thousands 
of tons of water which fall during a rainy day are stored, and also 
how precipitation takes place. In Plymouth the hot Return 
Trades come to us laden with vapour, and on mixing with the 
cold air over the land a first precipitation ensues. Then, as the air 
still containing a part of its vapour ascends the hills of Dartmoor, 
a portion of the pressure from the superincumbent air is taken off, 
and the air expands; but in so doing it must get heat from some 
place, and as it can derive it from no other source, it draws on 
itself for the supply, and thus cold is produced. Hence hillsides 
exposed to prevailing equatorial winds are always the localities of 
great rainfall; see Sleathwaite, in Cumberland; and the Western 
Ghauts, in India. I say hillsides, because if the hills be high the 
tops are frequently above the rain-bearing clouds, and then less 
rain would fall on the summit of the hill than on its sides. Very 
frequently, at 8 a.m., and even throughout the day, I find my wet 
and dry bulbs indicating the same temperature, thus showing the 
atmosphere to be completely saturated ; and occasionally I have 
seen the wet bulb as much as #° above the dry bulb, but this has 
been only after and during warm, drizzling, foggy weather. On 
June 22nd, 1865, and May 20th, 1880, I found 15° between the 
readings of the bulbs with relative humidities of 41 and 48°6. 
These are the least humidities I have observed in Plymouth, and 
each was after several days of continuous easterly winds. In 
June I find, as a rule, the least relative humidity, and in 
November the greatest. 
My rain gauge is by Casella, of the Snowdon pattern ; is 8 inches 
in diameter, and placed on my office, before referred to, 9 feet 
2 inches above the level of the ground, and 75 feet above mean 
sea level. From its height above the ground I lose from seven to 
eight per cent. of the rainfall; but what is worse, I am sheltered 
by trees from east-by-south to south-by-east, which cuts off a very 
large proportion of any rain which may fall when the wind blows 
between these points. ‘Towards the north-west there is another 
house; but as the top makes an angle of less than 40° with the 
horizontal line through the top of the gauge, I have no fear of 
losing much rain from that quarter, because, from the Rotherham 
