Page:A biographical dictionary of eminent Scotsmen, vol 5.djvu/119

Rh involved so much that was new and opposite to the opinions generally received. The description which it contains of the phenomena of geology, suppose in the reader too great a knowledge of the things described. The reasoning is sometimes embarrassed by the care taken to render it strictly logical, and the transitions, from the author's peculiar notions of arrangement, are often unexpected and abrupt. These defects, run more or less through all Dr Hutton's writings, and produce a degree of obscurity astonishing to all who knew him, and who heard him every day converse, with no less clearness and precision than animation and force." In the same volume of the Transactions appeared a paper by him, " A Theory of Rain," which he afterwards published in his " Physical Dissertations." Having long studied meteorology with great attention, this ingenious theory attracted almost immediate notice, and was valued for affording a distinct notion of the manner in which cold acts in causing a precipitation of humidity. It met, however, from M. De Luc with a vigorous and determined opposition; Dr Hutton defended it with some warmth, and the controversy was carried on with much sharpness on both sides.

In his observations in meteorology, he is said to be the first who thought of ascertaining the medium temperature of any climate by the temperature of its springs. With this view he made a great number of observations in different parts of Great Britain, and found, by a singular enough coincidence between two arbitrary measures quite independent of each other, that the temperature of springs along the east coast of this island varies a degree of Fahrenheit's thermometer for a degree of latitude. This rate of change, though it cannot be general over the whole globe, is probably not far from the truth for all the northern parts of the temperate zone. In explaining the diminution of temperature as we ascend in the atmosphere, Dr Hutton was much more fortunate than any other of the philosophers who have considered the same subject. It is well known that the condensation of air converts part of the latent into sensible heat, and that the rarefaction of air converts part of the sensible into latent heat; this is evident from the experiment of the air gun, and from many others. If, therefore, we suppose a given quantity of air to be suddenly transported from the surface to any height above, it will expand on account of the diminution of pressure, and a part of its heat becoming latent it will be rendered colder than before. Thus, also, when a quantity of heat ascends by any means whatever from one stratum of air to a superior stratum, a part of it becomes latent, so that an equilibrium of heat can never be established among the strata; but those which are less, must always remain colder than those which are more compressed. This was Dr Hutton's explanation, and it contains no hypothetical principle whatsoever. After those publications already mentioned had appeared, he resolved to undertake journeys into different parts of Scotland, in order to ascertain whether that conjunction of granite and schistus, which his theory supposed, actually took place. His views were first turned towards the Grampians, which the duke of Athol learning, invited him to accompany him during the shooting season into Glentilt, a tract of country situated under these mountains. On arriving there, he discovered in the bed of the river Tilt, which runs through that glen, many veins of red granite traversing the black micaceous schistus, and producing by a contrast of colour an effect that might be striking even to an unskilful observer. So vivid were the emotions he displayed at this spectacle, that his conductors never doubted his having discovered a vein of gold or silver. Dr Hutton has described the appearances at that spot, in the third volume of the Edinburgh Transactions, p. 79, and some excellent drawings of the glen were made by Mr Clerk, whose pencil was not less valuable in the sciences than in the arts.