ICE 



ICE 



bles are driven out, and the ice is much 

 clearer. 



Ice is considerably lighter than water, 

 namely, about one-eighth part ; and this 

 increase of dimensions is acquired with 

 prodigious force, sufficient to burst the 

 strongest iron vessels, and even pieces of 

 artillery. It does not arise from the ex- 

 trication of the gases ; for the refractive 

 power of ice is less than that of water, 

 as Dr. Hooke long ago shewed, and has 

 since been confirmed by Wollaston. 



M. Prevost observes, that congelation 

 takes place much more suddenly than the 

 opposite process of liquefaction ; and 

 that, of course, the same quantity of heat 

 must be more rapid^ extricated in freez- 

 ing, than is absorbed in thawing; that 

 the heat thus extricated being disposed 

 to fly off in all directions, and little of it 

 being retained by the neighbouring bo- 

 dies, more heat is lost than is gained by 

 the alteration: so that, where ice has once . 

 been formed, its production is in this 

 manner redoubled. This circumstance 

 must occur whenever it freezes, that is, 

 on shore, in latitudes above 35 degrees ; 

 and it appears, from 30 degrees to the 

 pole, the land is somewhat colder than the 

 sea, and the more as it is farther distant 

 from it ; and nearer the equator the land 

 is warmer than the sea : but the process 

 of congelation cannot, by any means, be 

 the principal cause of the difference, and 

 it is probable that the different capacity 

 of earth and water for heat is materially 

 concerned in it. 



Since the atmosphere is very little heat- 

 ed by the passage of the sun's rays 

 through it, it is naturally colder than the 

 earth's surface ; and for this reason the 

 most elevated tracts of land, which are 

 | the most prominent, and the most expos- 

 1 ed to the effects of the atmosphere, are 

 always colder than situations near the 

 i level of the sea. 



The northern hemisphere is somewhat 

 i warmer than the southern, perhaps be- 

 j cause of the greater proportion of land 

 that it contains, and also in some measure 

 | on account of the greater length of its 

 summer than that of the southern ; for 

 although, as it was long ago observed by 

 Simpson, the different distance of the-sun 

 compensates precisely for the different 

 velocity of the earth in its orbit, with re- 

 spect to the whole quantity of heat re- 

 ceived on either side of the equinoctial 

 points, yet M. Prevost has shewn that, in 

 all probability, the same quantity of heat 

 must produce a greater effect when it is 

 more slowly applied ; because the por- 

 tion lost by radiation from the heated bo- 



dy is greater as the temperature is high- 

 er. Since, therefore, on account of the 

 eccentricity of the earth's orbit, the north 

 pole is turned towards the sun seven or 

 eight days longer than the south pole, the 

 northern winters must be milder than the 

 southern; yet the southern summers, 

 though shorter, ought to be somewhat 

 warmer than the northern ; but, in fact, 

 they are colder, partly perhaps from the 

 much greater proportion uf sea, which in 

 some degree equalizes the temperature, 

 and partly for other reasons. The com- 

 parative intensity of the southern sum- 

 mer and winter is not exactly known ; 

 but in the island of New Georgia, the 

 summer is said to be extremely cold. 



The northern ice extends about 9 

 from the pole; the southern 18 or 20; 

 in some parts even 3l> ; and floating ice 

 has occasionally been found in both he- 

 mispheres as far as 40 from the poles, 

 and sometimes, as it has been said, even 

 in latitude 41 or42. Between 54 and 

 60 south latitude, the snow lies on the 

 ground, at the sea-side, throughout the 

 summer. The line of perpetual conge- 

 lation is three miles above the surface at 

 the equator, where the mean heat is 84; 

 at Tenetiffe, in latitude 28, two miles ; 

 and in the latitude of London, a little 

 more than a mile; and in latitude 8G a 

 north only 1,200 feet. At the pole, ac- 

 cording to the analogy deduced by Mr. 

 Kirwan, from a comparison of various ob- 

 servations, the mean temperature should 

 be 31. In London, the mean tempera- 

 ture is 50 ; at Rome, and at Montpelier, 

 a little more thun 60 ; in the island of 

 Madeira 70 ; and in Jamaica 80. 



IcE-Aozwe, a building 1 contrived to pre- 

 serve ice for the use of a family in the 

 summer season. Ice-houses are more ge- 

 nerally used in warm countries than with 

 us, particularly in Italy, where the mean- 

 est person, who rents a house, has hi* 

 vault or cellar for ice. However, as ice 

 is much more used in England than it was 

 formerly, it may not be amiss to give 

 some direction for the choice of their si- 

 tuation, for the manner of building them, 

 and for the management of the ice. 



As to the situation, it ought to be pla- 

 ced upon a dry spot of ground, because, 

 wherever there is moisture, the ice will 

 melt ; therefore, in all strong lands which 

 retain the wet, too much pains cannot be 

 taken to make drains all round them. 

 The place should also be elevated, and 

 and as much exposed to the sun and air 

 as possible. 



As to the figure of the building, that 

 may be according to the fancy of the own- 



