76. There have been many speculations as to the condition of the interior of the earth. Some have inferred that the external crust of the globe incloses a fluid or molten mass; others think it more probable that the interior is solid, but contains scattered throughout its bulk, especially towards the surface of the earth, irregular seas of molten matter, occupying large vesicles or tunnels in the solid honey-combed mass. At present, the facts known would appear to be best explained by the latter hypothesis. All that we know from observation is, that the temperature increases as we descend from the surface. The rate of increase is very variable. Thus, in the Artesian well at Neuffen, in Würtemberg, it was as much as 1° F. for every 19 feet. In the mines of Central Germany, however, the increase is only 1° F. for every 76 feet; while in the Dukinfield coal-pit, near Manchester, the increase was still less, being only 1° F. in 89 feet. Taking the average of many observations, it may be held as pretty well proved that the temperature of the earth's crust increases 1° for every 50 or 60 feet of descent after the first hundred. 77. The crust of the earth is subject to certain movements, which are either sudden and paroxysmal, or protracted and tranquil. The former are known as earthquakes, which may or may not result in a permanent alteration of the relative level of land and sea; the latter always effect some permanent change, either of upheaval or depression. 78. _Earthquakes_ have been variously accounted for. Those who uphold the hypothesis of a fluid interior think the undulatory motion experienced at the surface is caused by movements in the underlying molten mass--an earthquake being thus 'the reaction of the liquid nucleus against the outer crust.' By others, again, earthquakes are supposed to be caused by the fall of large rock-masses from the roofs of subterranean cavities, or by any sudden impulse or blow, such as might be produced by the cracking of rocks in a state of tension, by a sudden volcanic outburst, or sudden generation or condensation of steam. In support of this latter hypothesis, many facts may be adduced. The undulatory motion communicated to the ground during gunpowder explosions, or by the fall of rocks from a mountain, is often propagated to great distances from the scene of these catastrophes, and the phenomena closely resemble those which accompany a true earthquake. When the level of a district has been permanently affected by an earthquake, the movement has generally resulted in a lowering of the surface. Thus, in 1819, the Great Runn of Cutch, in Hindustan, was depressed over an area of several thousand square miles, so as during the monsoons to become a salt lagoon. Occasionally, however, we find that elevation of the land has taken place during an earthquake. This was the case in New Zealand in 1855, when the ground on which the town of Wellington stands