Essays and addresses / by professors and lectures of the Owens College, Manchester.

Date:
1874
Catalogue details

Licence: Public Domain Mark

Credit: Essays and addresses / by professors and lectures of the Owens College, Manchester. Source: Wellcome Collection.

    92/584 page 71
    III.] SOLAR PHYSICS. 71 in the earth's atmosphere to be very nearly saturated with aqueous vapour, that is to say just a Httle above the dew-point ; while, at the same time, it is losing heat with extreme slowness, so that, if left to itself, it would be a long time before moisture was deposited. Now, such a stratum is in an extremely delicate state of molecular equilibrium, and the dropping into it of a small crystal of snow would determine a change of state, just as it would if dropped into water cooled below the freezing-point, only from a different cause. For what would happen ? The snow would cool the air around it, and thus moisture would be deposited around the snow-flake in the shape of fine mist or dew. Now, this deposited mist or dew being a liquid, and giving out all the rays of heat possible to its temperature (Art. 4), would send its heat into empty space much more rapidly than the saturated air ; and therefore it would become colder than the air around it. Thus more air would be cooled, and more mist or dew de¬ posited ; and so on until a complete change of condition should be brought about. Now, in this imaginary case, the tiniest possible flake of snow has pulled the trigger, as it were, and made the gun go oíT,—has changed completely the whole arrangement that might have gone on for some time longer as it was, had it not been for the advent of the snow-flake. 16. We thus see how in our atmosphere the presence of a condensable liquid adds an element of violence, and also of abruptness, amounting to incalculability, to the motions which take place. But this must not be understood to imply that we cannot come to any true knowledge of terrestrial meteorology. On
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