4.0 
A THE GEELONG NATURALIST. 
vesicle theory is that rainbows are never formed on clouds or 
fogs as they would be—according to the undulatory theory of 
light—if they consisted of drops or globules of water. , Such 
men as Herschel and Tyndall, however, consider that this 
might be explained on other grounds, and hold that clouds 
consist of minute drops. 
In whatever state the particles of a cloud really exist, it is 
certain that the fall of rain depends on increased condensa- 
tion. The causes producing such condensation have been 
thus summed up by Professor Nichol : 
(1) The cooling of clouds through the effect of radiation 
from them. 
(2) The mingling of vapours at different temperatures— 
` a mingling effected by the agency of the winds. 
(3) The rising of vapours towards colder strata of the 
atmosphere. 
(4) The accumulation and impinging of masses of vapour 
. against the same object. H 
(5) The transfer from the equator towards the poles of 
large masses of moisture-laden air by means of the 
upper or counter trade-winds. 
When we consider all these causes we see that the produc- 
tion of rain is no simple matter; and then when we, in 
addition, consider the great variety of circumstances which 
may further affect these causes—as the latitude of a place, 
the elevation above sea-level, the proximity to thé sea, the 
seasonal variations, the prevailing winds, the configuration of 
the surrounding surface—it will be evident that meteorologists 
may well be perplexed by the very complex set of agencies 
acting in the production of rain, and so fail—as they have 
hitherto done, notwithstanding the prophecies of a N. S. W. 
astronomer—to interpret any save the most general laws. 
I will briefly notice some of these general laws; but 
before doing so I would have you carefully remember that all 
air is capable of keeping aqueous vapour in suspension; the 
higher the temperature of the air the more vapour it can keep 
in suspension. Also, when air at any temperature has as 
much vapour as it can possibly keep suspended at that tem- 
perature, it is said to be saturated, and when air is saturated 
on any or all of the five causes of cooling which I have 
mentioned coming into play, condensation immediately takes 
place. 
(To be continued.) 
