Earth, exposed naked to the sun's rays, absorbs a good portion of them; but, being an opaque body, those rays penetrate to a small depth only. Its surface, by this accumulation of absorbed rays, becomes considerably heated. The residue of the rays are reflected into the air resting on that surface. This air, then, is warmed, 1, by the _direct_ rays of the sun; 2, by its _reflected_ rays; 3, by _contact_ with the heated surface. A forest receiving the sun's rays, a part of them enters the intervals between the trees, and their reflection upwards is intercepted by the leaves and boughs. The rest fall on the trees, the leaves of which being generally inclined towards the horizon, reflect the rays downwards. The atmosphere here, then, receives little or no heat by reflection. Again, these leaves having a power of keeping themselves cool by their own transpiration, they impart no heat to the air by contact. Reflection and contact, then, two of the three modes before-mentioned of communicating heat, are wanting here; and of course, the air over a country covered by forest must be colder than that over cultivated grounds. The sea being pellucid, the sun's rays penetrate it to a considerable depth. Being also fluid, and in perpetual agitation, its parts are constantly mixed together; so that, instead of its heat being all accumulated in its surface, as in the case of a solid opaque body, it is diffused through its whole mass. Its surface, therefore, is comparatively cool, for these reasons, to which may be added that of evaporation. The small degree of reflection which might otherwise take place, is generally prevented by the rippled state of its surface. The air resting on the sea, then, like that resting on a forest, receives little or no heat by reflection or contact, and is therefore colder than that which lies over a cultivated country.