nights it is the latent heat of the vapour condensed into dew. This vapour is taken chiefly from the air engaged among the stems and leaves, which, in the case at least of fine grass, is all nearly at the same temperature as the leaves; the temperature of the surface of these being of course rigorously the same as that of the air in contact with them. Thus the temperature of the leaves can never go below the dew-point of the air touching them, and any cooling which they experience after dew begins to deposit upon them is only equal to the lowering of the dew-point, occasioned by the amount of drying experienced by the air in consequence of the condensation of vapour out of it.

Clouds, as remarked first by Prévost, being practically opaque, prevent the surface of the earth from tending by radiation to a lower temperature than their own, which, unless they are very high, is generally not much colder than the dewpoint of the lower air, but is at all events in general sufficiently warm to prevent the finest blades of grass from acquiring any very sensible dew, or to allow the general temperature of grass

and the air engaged among it, even on the stillest night, to sink as low as the dew-point. Thus either clouds, by their counter radiation, or wind, by mixing a comparatively thick stratum of air with that next the earth, keep the grass and delicate parts of other plants from sinking to the dewpoint. When there is not enough of clouds and wind to afford this degree of protection, dew begins to form, and by preventing the temperature of any leaf or flower from sinking below the dew-point, saves them all from destruction, unless, as when hoar-frost appears, the dew-point itself is below the freezing-point.

[Added December 15, 1892.]—Thus when neither clouds, nor wind blowing among the plants, suffice to protect them from sinking to the dewpoint, the temperature to which leaves, flowers and grass sink is lower the dryer is the air, not because dry clear air is more diathermanous than moist clear air, but because the dew-point is lower the dryer is the air.

1 Tyndall, "On Terrestrial Radiation," Proc. Roy. Soc., February, 1883.

THE

"DOCTRINE OF UNIFORMITY"

IN GEOLOGY BRIEFLY

REFUTED.

[Paper read before the Royal Society of Edinburgh, December 18, 1865.]

THE "Doctrine of Uniformity" in Geology, as held by many of the most eminent of British Geologists, assumes that the earth's surface and upper crust have been nearly as they are at present in temperature, and other physical qualities, during millions of millions of years. But the heat which we know, by observation, to be now conducted out of the earth yearly is so great, that if this action had been going on with any approach to uniformity for 20,000 million years, the amount of heat lost out of the earth would have been about as much as would heat, by 100° Cent., a quantity of ordinary surface rock of 100 times the earth's bulk. (See calculation appended.) This would be

more than enough to melt a mass of surface rock equal in bulk to the whole earth. No hypothesis as to chemical action, internal fluidity, effects of pressure at great depth, or possible character of substances in the interior of the earth, possessing the smallest vestige of probability, can justify the supposition that the earth's upper crust has remained nearly as it is, while from the whole, or from any part, of the earth, so great a quantity of heat has been lost.

APPENDIX.

ESTIMATE OF PRESENT ANNUAL LOSS OF HEAT FROM THE EARTH.

LET A be the area of the earth's surface, D the increase of depth in any locality for which the temperature increases by 1° Cent., and k the conductivity per annum of the strata in the same locality. The heat conducted out per annum per square foot

k

of surface in that locality is Hence, if we give

D'

k and D proper average values for the whole upper crust of the earth, the quantity conducted out

across the whole earth's surface per annum will

be

kA The bulk of a sphere being its surface D multiplied by of its radius, the thermal capacity of a mass of rock equal in bulk to the earth, and of specific heat s per unit of bulk is Ars. Hence

3k

Dr's

is the elevation of temperature which a quantity of heat equal to that lost from the earth in a year, would produce in a mass of rock equal in bulk to the whole earth. The laboratory experiments of Peclet; Observations on Underground Temperature in three kinds of rock in and near Edinburgh, by Forbes ; in two Swedish strata, by Ångström, and at the Royal Observatory, Greenwich, give values of the conductivity in gramme-water units of heat per square centimetre, per 1o per centimetre of variation of temperature, per second, from 002 (marble, Peclet) to 0107 (sandstone of Craigleith quarry, Forbes); and *005 may be taken as a rough average. Hence, as there are 31,557,000 seconds in a year, we have k=005 × 31,557,000, or approximately 16× 101. The thermal capacity of surface rock is somewhere about half that of equal bulk of water; so that we may take s='5. And the increase of temperature downwards may be taken as roughly averaging 1° Cent. per 30 metres; so, that, D= 3000 centimetres. Lastly, the earth's quadrant being according to the first foundation of the French