CHAPTER II 

 PRECIPITATION 



i. ITS CAUSES. We have next to consider the various questions 

 connected with precipitation. The preliminary observations have 

 given some facts which can be utilised as a starting-point. We 

 have seen that the road dries quicker on some days than others, 

 quickest of all on warm windy days with sunshine. We have taken 

 advantage of such trivialities, from the strictly scientific standpoint, 

 as the family washing, and we have learnt that the clothes dry 

 quickest on such days, slowest on damp, sunless, windless days ; we 

 have seen also that on the same day they dry quicker the more fully 

 they are exposed to the air, and we have watched the props being 

 adjusted with the object of raising the line as high as possible in 

 the air. The clothes are put out wet, they are taken in dry where 

 does the moisture go ? Not into the clothes-line, not into the ground, 

 and therefore it must be into the air, though we can see no differ- 

 ence in the air. Therefore we conclude that the air can take up 

 moisture, and that when so taken up the moisture is invisible. 

 But if the clothes, other things being equal, dry quicker the greater 

 the movement of the air, then it is probable that the air can only 

 take up a limited amount of moisture. What becomes of all the 

 moisture that the air takes up ? 



The answer to this question should be given gradually, in 

 making use of all the simple observations which present themselves 

 in the course of daily life. In the winter the white cloud of vapour 

 which one sees issuing from the nostrils of horses standing in the 

 frosty air ; the similar cloud which escapes from our nostrils in the 

 open, while it is not visible in the warm room ; the cloud which 

 forms on the outside of a glass of water taken from a deep well or 

 a cold spring in summer ; the similar cloud outside a glass of iced 

 lemonade ; the steam on the windows of a railway carriage or 

 tram car ; the similar condensation on the windows of the in- 



