and of the Planetary Spaces. 59 



In general, the diurnal inequalities are insensible, at the depth of 

 one metre, (2) and the annual inequalities disappear at the dis- 

 tance of twenty metres from the surface ; while at about one 

 third that distance this is reduced to an inequality of which the 

 period embraces the entire year. At the depth of six or eight 

 metres the temperature offers, then, but one maximum and one 

 minimum during the year ; which have an interval of six months 

 between them, corresponding to the epochs of greatest and least 

 solar heat. Beyond a depth of about twenty metres the temper- 

 ature no longer varies with the seasons ; or, at least, it can only 

 experience secular variations which have not yet been observed. 



Upon each vertical the inequalities of temperature, both daily 

 and annual, are accompanied by an ascendant or descendant flux 

 of heat, the quantity and direction of which varies with the time 

 and the depth. The extent of these inequalities and of this flux 

 of heat is not the same, in all latitudes ; at the equator, for ex- 

 ample, the annual inequalities mostly disappear, and consequently 

 the temperature should there be very nearly constant, at a much 

 less depth than at any other place. 



******* 



Near the surface of the earth the mean temperature due to the 

 solar heat varies with the obliquity of the ecliptick, which enters 

 into the function I have designated by Q-. This secular inequal- 

 ity, like those which are daily and annual, is attended with a va- 

 riation in the direction of depth, which we are not able to deter- 

 mine with accuracy, in default of knowing the expression of the 

 obliquity of the ecliptick, in the function of time ; but the data 

 we have of the extreme slowness of the displacement of the 

 ecliptick, and of the minuteness of this displacement suffice to 

 show that the variations of terrestrial temperature, arising from 

 this cause, are very feeble, and can therefore but slightly influ- 

 ence the observed increase of temperature, at augmented depths. 

 Fourier, and afterwards Laplace, attributed this phenomenon to 

 the original heat which the earth has still preserved, and which 

 they suppose to increase constantly, from the surface to the cen- 

 tre of the globe ; so that, while at the centre there is a tempera- 

 ture excessively elevated, yet near the surface the heat, from this 

 cause, is scarcely perceptible : that in virtue of this primitive 



(2) The metre is =3.2808992 English feet. 



