Mffeas of Cold on n Lah of Prtjh Wattr. j5^ 



But to return to our lake. As foon as the water In cooling has arrived at the temperature 

 «f about 40°, as at that temperature it ceafes to be farther condenfed, its internal motion 

 ceafes, and thofe of its particles which happen to be at its furface remain there ; and after 

 be.ng cooled down to the freezing point they give off their latent heat, and ice begins to be 

 formed. '' 



As foon as the furface of the water is covered with ice, the communication of heat from 

 the water to the atmofphereis rendered extremely flow and difficult; for ice, being a bad 

 conductor of heat, forms a very warm covering to the water ; and moreover it prevents the 

 water from being agitated by the wind. Farther, as the temperature of the ice at its 

 lower furface is always very nearly the fame as that of the particles of liquid water with 

 which it is in contaft (the warmer particles of this fluid, in confequcnce of their greater 

 fpecific gravity, taking their places below), the communication of heat between the water 

 and the ice is neceffarily very flow on that account. 



As foon as the upper furface of the ice is covered with fnow (which commonly happens 

 foon after the ice is formed), this is an additional and very powerful obftacle to prevent 

 the efcape of the heat out of the vcater ; and though the molt intenfe cold may reign in 

 the atmofphere, the increafe of the thicknefs of the ice will be very flow. 



During this time the mafs of water which remains unfrozen will lofe no part of its heat ; 

 on the contrary, it will continually be receiving heat from the ground. This heat, which 

 is accumulated In the earth during the fummer, will not only ferve in fome meafure to 

 replace that which Is communicated to the atmofphere through the ice, and prevent its 

 teing furnllhed at the expence of the latent heat of the water in contaft with its furface ; 

 but, when the temperature of the air is not much below that of freezing, this fupply of 

 heat from below will be quite fufficient to replace that which the air carries off, and the 

 thicknefs of the ice will not Increafe. 



Whenever the temperature of the air is not a£lually colder than freezing water, the 

 heat which rifes from the bottom of the lake will be ail employed in melting the ice at iis 

 under furface, and diminilhing its thicknefs. 



It will indeed frequently happen, when the ice Is very thick, and efpecially when Its 

 upper furface is covered with deep fnow, that the melting of the ice at its under furface 

 will be going on, when the temperature of the atmofphere is conCderably below the freet- 

 ing point. 



As the particles of the water, which, receiving heat from the ground at the bottom of 

 the lake, acquire a higher temperature than that of 40 degrees (and being expanded, and 

 becoming fpecifically lighter by this additional heat, rife £0 the upper furface of the fluid 

 water, and give off their fenfible heat to the under furface of the ice), never return to the 

 bottom, this communication of the heat which exhales from the earth produces very little 

 motion in the mafs of the water ; and this circumdance is no doubt very favourable to the 

 prefervation of the heat of the water. 



When a ftrong wind prevails, and the furface of the water is much agitated. Ice is not 

 formed, even though the whole mafs of water fliould, by a long continuance of cold 

 weather, have been previoufly cooled down to that point to which it is ncoeflary that it 

 ftiould be brought, in order that its internal motions may ccafe, and that it may be difpofcd 

 to congeal. For though the particles at and near tlw furface may no longer have any ten- 

 VoL. I. — March 1798. 4D dcncy 



