TERRESTRIAL HEAT. 



lakes. 32. Thermal condition of a frozen sea. 33. Process of 

 thawing. 34. Depth of stratum of constant temperature. 35. 

 Superficial agitation extends only to a small depth. 36. Great 

 utility of the state of maximum density. 37. Variations of tempe- 

 rature of the air. 38. Interchange of Equatorial and Polar waters 

 39. Polar ice. 40. Ice-fields. 41. Icebergs. 42. Their forms and 

 magnitude. 43. Sunken icebergs. 44. Curious effects of their 

 superficial fusion. 45. Depth of Polar Seas. 46. Cold of Polar 

 regions. 



1. OF all physical agents, heat is the most intimately connected 

 with the terrestrial economy, the most important to the well-being 

 of the organised tribes which inhabit the earth, and that upon the 

 play of which the most remarkable revolutions which our planet 

 has undergone have been more or less dependent. Since, in some 

 future numbers of this series, we propose to explain these revo- 

 lutions, and the traces they have left upon the crust of the globe, 

 it will be useful to supply at present some preliminary information 

 as to the laws which regulate the distribution of heat, and the 

 periodical vicissitudes of temperature, on and below the surface of 

 the earth, and in the superior strata of the atmosphere. 



2. The superficial temperature of the earth varies with the 

 latitude, gradually decreasing in proceeding from the equator 

 towards the poles. 



It also varies with the elevation of the point of observation, 

 decreasing in proceeding to heights above the level of the sea, and 

 varying according to certain conditions below that level, but in 

 all cases increasing gradually for all depths below a certain 

 stratum, at which the temperature is invariable. 



At a given latitude and a given elevation the temperature varies 

 with the character of the surface, according as the place of 

 observation is on sea or land ; and if on land, according to the 

 nature, productions, or condition of the soil, and the accidents of 

 the surface, such as its inclination or aspect. 



3. At a given place the temperature undergoes two principal 

 periodic variations, diurnal and annual. 



The temperature falling to a minimum at a certain moment 

 near sunrise, augments until it attains a maximum, at a certain 

 moment after the sun has passed the meridian. The temperature 

 then gradually falls until it returns to the minimum in the 

 morning. 



This diurnal thermometric period varies with the latitude, the 

 elevation of the place, the character of the surface, and with a 

 great variety of local conditions, which not only affect the hours 

 of tie maximum, minimum, and mean temperatures, but also the 

 difference between the maximum and Tm'nJTmim, or the extent of 

 the variation. 

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