60

TREE CALORIC.

temperature to some little depth, but it can de-scend no further than the agitation extends.

EMILY.

But \yhen the atmosphere is colder .than thelake, the colder surface of the water will descendfor the very reason that the warmer will not?

MRS. B.

Certainly ! and it is on this account that nei-ther a lake, nor any body of water whatever, canbe frozen until every particle of the water has jrisen to the surface to give off its caloric to the jcolder atmosphere ; therefore the deeper a body jof water is,'the longer will be the time it requires jto be frozen r j

EMILY.

But if the temperature of the whole body ofwater be brought down to the freezing point, whyis only the surface frozen ?

MRS. B.

The temperature of the whole body is lowered,but not to the freezing point? The diminution ofheat, as you know, produces a contraction in thebulk of Huids, as well as of solids. This effect,however, does not take place in water below thetemperature of 40 degrees, which is 8 degrees