158 



METEOROLOGY. 



Meteorolo- mean of the extremes. The temperature of the water 

 was taken three times a-month, at equal intervals of 

 10 days ; and the following table exhibits the mean of 

 the three observations. 



Results. 



thermome 

 tert sunk 

 into the 

 earth. 



It appears from the above table, that the tempera- 

 ture of the ground, at the depth of 25 feet, arrives at 

 its minimum about the month of March, and at its ma- 

 ximum in October or November, according to circum- 

 stances afterwards to be noticed. The difference of the 

 highest and lowest monthly average in 1814 is 5. 6, and 

 in 1815, 5 9; but if we compare the single observa- 

 tions of these months, we. shall find that the range is 

 considerably greater. In 1814 it amounted to 6, and 

 in 1815, to 6.4. On the 20th of March 1816, it stood 

 at 40.4, and on the 20th of August in the same year, at 

 48. 4, being a variation of 8. 



Kiperi- These facts are calculated to throw considerable light 



ments with on the gradual diffusion of heat through the interior of 

 the ground. From experiments lately made with ther- 

 mometers placed at various depths from one to eight 

 feet, it is found that, during the summer season, a cur- 

 rent of heat is passing regularly, but slowly from the 

 surface downwards, as long as the mean temperature of 

 the atmosphere is higher than that of the interior ; and 

 that in winter the current flows upwards towards the 

 surface, to supply the deficiency produced by the ex- 

 ternal cold. In this way an equilibrium is gradually 

 established in the temperature of the earth at moderate 

 depths, and is found to take place twice a-year, viz. 

 about May and October. 



Results not F rom 8 '1 that we know of the nature and diffusion of 

 fairly esti- heat, it is impossible to doubt the existence of such a 

 mated. current as is here stated ; or its tendency in all cases to 

 move from the warmer towards the colder region. In 

 estimating its progress, however, a very important ele- 

 ment seems in general to have been overlooked, we 

 mean the influence of rain, in conveying heat or cold to 

 the- interior strata of the earth. The current alluded 

 to might perhaps be thought sufficient to account for 

 the fluctuations of temperature, a few feet below the 

 surface, but seems altogether inadequate lo explain the 

 changes which we have seen to take place at the depth 

 of 25 feet. A thermometers feet deep, does not arrive 

 at its highest and lowest temperatures till two months 

 after the extremes of the year ; yet we find water at 

 the depth of 25 feet reaching its extremes in less than 

 three months. This fact, we think, can only be ex- 

 plained, by referring to the commonly received opinion 



respecting the origin of springs ; which ascribes them Meteorolo. 

 to water deposited from the atmosphere on the higher SY- 

 ground*, and passing through the earth as a filter, till, ' "" * " ~^ 

 being arrested by an impermeable stratum, it flows ^ ommu ni- 

 along the surface of that stratum, and bursts out in *'" f h 

 springs, or is intercepted by pits dug for the purpose, | n " ,.'",.' j'.. 

 The rain and melted snow of winter, being cooled down fluenced by 

 on their first entering the ground, far below the mean rain. 

 temperature of the interior of the globe, successively ab- 

 stract, from the strata through which they pass, a por- 

 tion of caloric ; and though the quantity of water is not 

 such as to cool the whole mass more than a few degrees 

 below the mean temperature, yet it is obvious that the 

 diminution must continue till the surface again ap- 

 proaches the temperature of the interior. This equili- 

 brium will take place towards the middle or end of 

 March, as the surface of the earth is then generally 

 within a few degrees of the mean annual temperature. 

 It is found, accordingly, that the temperature of the in- 

 terior, as indicated by the pump water, is actually a mi- 

 nimum about the middle of March. From that period, 

 it gradually increases, and appears to reach its mean 

 about the middle of June. The rain, and dews, how- 

 e\er, of the succeeding months, being still at a compa- 

 ratively high temperature, communicate additional heat 

 to the strata beneath, and must continue to do so till 

 the surface of the ground again descend towards the 

 temperature to which the interior has risen. This 

 point, for the reasons already stated, will be a few de- 

 grees above the mean, and of course the equilibrium 

 ought to take place about the beginning of October, as 

 the temperature of the atmosphere is -then generally 

 within a few degrees of the mean annual temperature. 

 It is obvious, however, that the ground, to the depth of 

 several feet, may, from the accumulation of the sun's 

 rays, be preserved at a higher temperature than the 

 mean, even after that of the atmosphere has sunk consi- 

 derably lower. This will happen to a certain extent 

 perhaps every year ; but especially in warm and dry 

 seasons. Making an allowance, then-fore, for this cir- 

 cumstance, the equilibrium between the surface and the 

 interior, may be expected to take place about the end 

 of October, which agrees exactly with observation, the 

 pump-water being then a maximum. From this pe. 

 riod the temperature decreases, and reaches its medi. 

 um again towards the end of December. 



But this view of the subject is still farther illustrated 

 by several circumstances of a more particular nature. 

 During the whole of the month of August (1815,) when 

 very little rain fell, the temperature of the water was 

 stationary at 46. 8. In the course of September, and 

 the first 15 days of October, when the quantity of rain 

 considerably exceeded the evaporation, the temperature 

 rose to 4<7.8, being 1 in about fifty days ; and between 

 the 15th and 25th of October, during which time there 

 fellupwards of two inches of rain, with lit tic evaporation, 

 it rose to 48.8, being 1" in about one-fifth of the pre- 

 ceding interval. (See Annals of Phil. May, 18)6.) 

 During the month of July 18 1 6, though very wet, the 

 temperature ranged only between 4-5" and 45.7, the in- 

 terior having then arrived at its mean anmu.l tempera- 

 ture ; but, in the course of 13 days in August, when a 

 great deal of rain fell, the water rose from -tb" to 48.4. 

 It is easy to see from these facts, that the interior of the 

 earth may reach its extreme temperature sooner or later 

 one year than another, according to the quantity of rain, 

 and the periods at which it falls; and it is equally ob- 

 vious, that every register of thermometers below the 

 surface, ought to be accompanied with a corresponding 

 register of the rain gage. 



