542 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



the meadow, not only because its radiation into space is wholly 

 or partly intercepted, but also because the medium surrounding 

 it is less tranquil ; and the union of these two causes M'ill pro- 

 duce the marked difference which is found, during calm and 

 clear nights, in passing from the open field to a wood or from 

 the wood to the fields. 



When we reflect on the numerous inequalities of temperature 

 resulting, as well by night as day, from the nature, the form, the 

 exposed state or the culture of the soil, we soon become con- 

 vinced that absolute equilibrium never exists in the atmosphere ; 

 what we call a calm atmosphere is, properly speaking, only a less 

 violent agitation of it. It is in consequence of this incessant 

 perturbation of the atmosphere, that the stratum of air cooled 

 by contact with plants and the soil gradually mingles itself with 

 the upper strata, even in the seasons of greatest apparent calm ; 

 moreover, the quantity of air condensed by contact with plants 

 will go on increasing upon the soil as the night advances, and 

 will attain greater and greater heights. Hence the origin of the 

 two facts discovered by Peclet and Dufay, namely, the nocturnal 

 inversion of the atmospheric temperature, which in calm and 

 clear weather diminishes instead of increasing (as it does in the 

 day-time) on approaching the soil, and the precipitation of dew 

 becoming retarded on a substance isolated or surrounded by 

 plants in proportion as its distance from the earth's surface is 

 increased. Hence the limits which we have supposed between 

 the two strata, "upper" and " lower," will never be very distinct; 

 and the cold and humidity will diminish by insensible degrees 

 as we rise in the atmosphere. It will nevertheless be under- 

 stood, that the transition will be more or less abrupt according 

 to the nature of the soil and the time of observation ; and if the 

 air were coloured so as to be perceptible in the dusk, we should 

 see at night this colour become more marked near the earth's 

 surface up to a certain height, greater in proportion as the night 

 is further advanced, and thus form a kind of zone, of greater or 

 less magnitude and distinctness, which would follow the general 

 distribution of vegetation, attaining its maximum, of intensity on 

 meadows and fields clothed with low plants, growing thick and 

 close, and spreading on all sides as far as the furthest boundary 

 of the horizon. 



From all that precedes, it follows that Wells's principle with 

 regard to the formation of dew in virtue of the radiation of bodies, 



