ANNIVERSARY ADDRESS. 23 
reproduce the same effects. These phenomena are strictly in accord- 
ance with the known laws of atmospheric condensation, At the 
Equator the calm belt is supplied with air from the trades, which 
are almost saturated with moisture, and this air, resting on the 
ocean under a vertical sun, the saturation necessarily becomes com- 
plete. Now the series of effects we have just considered comes about 
‘in this way: the sun, as it gains power in the forenoon, heats the 
stratum of air which rests on the water, and gives rise to evap- 
oration from the sea; the moist heated air begins to rise rapidly ; " 
but so soon as its cooling, from elevation and expansion, reduces 
its temperature to the dew-point, then moisture is deposited as 
cloud, and the ascending rate, accelerated by this deposition, leads 
to increasing clouds above, and to rapid cooling down of the air 
below; for the heavy clouds stop the sun’s rays and throw all below 
them into shade and a much lower temperature ; hence the heavy 
rain which speedily brings down the moisture that had been carried 
up ; the cloud particles left after the heavy rain fall slowly down, 
and melt as they fall; the sun having meantime by his westerly 
course lost the morning’s power, the evaporation is not renewed, 
‘and the sun sets in a clear sky. 
This process carried on in the calm belts is just that which may 
be seen on a calm day, and I may mention here, as an illustration, 
a recent observation in Sydney. April 20, 1882, the morning was 
fine and bright and the air very moist ; at 9 a.m. the difference 
between the wet and dry bulbs was only 1-7°. It was perfectly 
calm all the morning, and there were only a few cirrus and small 
- eumulus clouds about at 9am. As the morning wore on the sun 
got very powerful, and the little cumuli grew into great ones, roll- 
ing out great masses from the top, and so forming a shade for the 
_ Clouds and earth below. The base of these clouds seemed to spread 
out into dense stratus, and about noon I could see that those 
_ in the W.S.W. were depositing rain, and now and then a down- 
Stroke of lightning, followed by low rumbling thunder. The 
Pea paabaes were rapidly, and, forming over the Observatory, ob- 
Be = and caused a sudden fall in the temperature, shown 
