1390. The cause why less rain falls in the first six months of the year than in the last six months is thus explained. The whole quantity of water in the atmosphere in January is usually about three inches, as appears from the dew point, which is then about 32ﾦ; now, the force of vapours of that temperature is 0.2 of an inch of mercury, which is equal to 2.8, or three inches of water. The dew point in July is usually about 58ﾦ or 59ﾦ, corresponding to 0.5 of an inch of mercury, which is equal to seven inches of water. Thus it is evident that, in the latter month, the atmosphere contains four inches of water more than in the former month. Hence, supposing the usual intermixture of currents of air, in both the intervening periods, to be the same, the rain ought to be four inches less in the former period of the year than the average, and four inches more in the latter period, making a difference of eight inches between the two periods, which nearly accords with the preceding observations.
