i88o.] 
On Water and Air . 
3i7 
ON WATER AND AIR* 
By John Tyndall, D.C.L., LL.D., F.R.S., 
Professor of Natural Philosophy at the Royal Institution 
of Great Britain. 
Lecture V. 
TRUST that you already know as well as I do that 
j|P the vapour of the ocean is raised by the sun into the 
atmosphere, and that when chilled it becomes preci- 
pitated as cloud, and that when the chilling goes far enough 
it is precipitated as snow. One sample of cloud manufac- 
ture I should like to bring to your attention, because it is a 
very interesting one ; and I should like you when you go to 
the Alps in future years, as I have no doubt many of you 
will, to be able to notice this phenomenon, and also to un- 
derstand its nature. You sometimes find a perfectly trans- 
parent air charged with aqueous vapour in the true invisible 
vaporous condition, blowing against a cold mountain-crest. 
On the side of the mountain upon which this air impinges 
all is perfectly clear and bright, but on the other side of the 
mountain you have a vast cloud banner drawn out, which is 
the aqueous vapour of this perfedtly transparent air preci- 
pitated into cloud by the cold crest of the mountain. I 
have seen it on many peaks frequently. On the Matter- 
horn, for instance, the air charged with aqueous vapour 
comes up from the plains of Italy; meeting with the 
mountain-crest it is chilled, and condenses on the farther 
side of the mountain, forming a large streaming cloud, 
which appears to be attached to the summit of the peak, 
with perfect steadiness, in spite of the strong wind that is 
blowing. This steadiness, however, is only in appearance, 
the cloud being constantly dissipated at its farther extremity, 
but as constantly renewed by the continuous supply of warm 
moist air which gives rise to it. 
We have now to consider the subject of the formation of 
glaciers, and as an example of a simple glacier will take the 
* Being a Course of Six Le&ures adapted to a Juvenile Auditory, delivered 
at the Royal Institution of Great Britain, Christmas, 1879. Specially re- 
ported for “ The Journal of Science.” 
