100 ASTRONOMYANDTHEBIBLE
below, sinks and gives place to the warmer water,
which is cooled and sinks in its turn, and thus, as
before, a circulation is established, which continues
untilthetemperature ofthewhole wateris reduced
to40°. Butatthis point,thecirculation isentirely
arrested; forinconsequenceofitssingular constitu-
tion,waterat39°islighterthanwaterat40°,andcon-
sequentlyremainsatthetop. Andsoitisasthetem-
peraturesinkstowardthefreezingpoint. Thecolder
thewater,thelighter itbecomes, andthemore per-
sistently itremainsatthe surface. Hence,although
theupperlayersofwatermaybereadilycooledtothe
freezingpoint, yet, in consequence of itspoor con-
ductingpower, the greatbodyof theliquidwill re-
mainatthetemperatureof40°.
"Thecoldatmosphereofwinteractsupontheponds
andlakesexactlyastheiceonthewaterinthepitcher.
Theyalsoarecooledfromthesurface,andacircula-
tionisestablishedbytheconstantsinkingofthechilled
wateruntilthetemperaturefallsto40°. Butatthis
point,stilleightdegreesabovethefreezingpoint,the
circulationstops. Thesurfacewater,asitcoolsbelow
thistemperature,remains atthetop,andinthe end
freezes; butthen comes intoplay still another pro-
visioninthepropertiesofwater. Mostsubstancesare
heavier intheir solid than intheir liquid state; but
ice,onthecontrary,is lighterthanwater,andthere-
forefloatsonitssurface. Moreover,asiceisavery
poorconductorofheat,itservesasaprotectiontothe
lake,sothatatthedepthofafew feet,atmost, the