and of the Planetary Spaces. 59 
In general, the diurnal inequalities are insensible, at the depth of 
one métre, (2) and the annual inequalities disappear at the dis- 
tance of twenty métres 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 
métres 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 métres 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 ddimeqppeetr 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 Ihave 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 métre is = 3.2808992 English feet. 
