It is this reflection which must always teach humility to the scientific student, even while he rejoices in the achievements of human patience and genius. He will not despair for he knows that great victories have been won: he will not grow arrogant, for he knows that he is still on the threshold of eternal truth. As Sir J. Herschel has justly said:--"He who has seen obscurities, which appeared impenetrable, in physical and mathematical science, suddenly dispelled, and the most barren and unpromising fields of inquiry converted, as if by inspiration, into rich and inexhaustible springs of knowledge and power, on a simple change of our point of view, or by merely bringing them to bear on some principle which it never occurred before to try, will surely be the very last to acquiesce in any dispiriting prospects of either the present or the future destinies of mankind; while, on the other hand, the boundless views of intellectual and moral, as well as material relations, which open to him on all hands in the course of these pursuits,--the knowledge of the trivial place he occupies in the scale of creation, and the sense continually pressed upon him of his own weakness and incapacity to suspend or modify the slightest movement of the vast machinery he sees in action around him, must effectually convince him that humility of pretension, no less than confidence of hope, is what best becomes his character."[79] The temperature of the terrestrial surface perpetually varies under the influence of local as well as general causes. Had this fact been known to the philosophers of antiquity, they would have taken advantage of it to liken the earth to an animal whose skin is more or less sensible of heat, not only according to the difference of the seasons, but according to the different hours of the day. The diurnal thermometrical variations are those which penetrate the least profoundly into the interior of the soil. At a depth of about five feet they cease to be perceptible. The maxima and minima of the year, however, can be detected at a sufficiently considerable depth. The limit descends as low as 80 to 100 feet. Below 100 feet, the terrestrial stratum is found invariable,--that is, inaccessible to the thermometrical changes of the atmosphere. It is remarkable that the temperature of this stratum differs but little from the mean annual temperature of the air, which, in the latitude of London, is 49°. * * * * *