THE EARTH’S INTERNAL HEAT.
No. 2. TO THE EDIXOB. Sir, —In ray last communication I hinted that I would send you a few lines re the question Is the interior of the earth molten or rigid ? ” I regret that I have been so long in doing so ; the long interval is caused by my absence from home, and a pressure of ministerial duties. It must be conceded that the earth was once in. a niolten state. Its shape (oblate spheriod) has been deemed sufficient proof of this. The notion that the earth js a perfect sphere, with the exception of a slight polar compression, is not quite correct; for while it is oblate at the poles it is also prolate at the equator, the same law causing both the equatorial protuberance and the polar compression. Although this bulging at the equator is almost inappreciable in diagrams yet considered pnysiographically it is of much greater importance , than is commonly supposed. Perhaps few men .can speak with greater authority upon this subject than Sydney B. J. Skertchley, F.G.S., of H.M. Geological Survey. He says, speaking of thus equatorial protuberance—“ 1 have estimated that it exceeds the volume of the entire land (measured from sea-level) as much as 18,000 times. ” The same writer remarks : “ Suppose the earth to be a sphere at rest and to- be covered with an uniform ocean. Then let a quantity of solid matter be taken from the Polar regions and piled about the equator, so as to bring the earth’s shape into its present form. This would be clearly equivalent to building a mountain over the equator 13£ miles high, or about 70,000 feet [69,960] and whose base is equal to the surface of the original sphere.” It is believed that molten matter rotating in space at the rate of the earth’s present rotation would take precisely the form just described. The following experiment was made by M. Plateau ; —A mass of oil was placed in & transparent liquid exactly of the same density as the oil, and it was found as long as the oil was at rest it took the form of a perfect sphere floating in the middle of the fluid like the earth in space, but when a motion of rotation was given to the oil the spherical form changed into a spheroidal form like the earth. The shape of the earth, together with several principles of geology and physical geography, is held conclusive evidence of the earth having been at some time molten.
It is also conceded. that the interior of the earth is at the present time intensely hot; so hot that “taking the increment of heat at 1° Fah, for every 60 feet, it is apparent that, other things being the same, a temperature will be attained at a moderate depth sufficiently high to melt every known rock. Thus, at about 35 miles from the surface, the heat would be sufficient to melt platinum the most refractory of metals.” I find that Professor Loomis, LL.D., reckons the average increment of heat in Europe to be 1° for every 52 feet. In Louisville, Kentucky, in a boring 2086 feet deep, 1° for every 76 feet. In Columbus, Ohio, in a boring 2575 feet deep, 1° for every 71 feet. The mean of these American localities being 1“ for every 73 feet Beete Jukes, F.R.S., and Sydney B. J. Skertchley, F.G.S., both, reckon a feneral average of 1° for 60 feet, J. forman Loekyer, F.R.S., F,8.A.5., &c., 1° (centigrade) for every 90 feet From the above it has been assumed that the interior of the earth must be molten. In the “Museum of Science and Art,” in one of the chapters upon popular geology, may be found a diagram showing a section through the earth, a thin black line marking the thickness of the crust of the earth, upon? which insignificant film wo are supposed to be living. Well may one writer speak of it as a “ perilous bomb. 1 ' I shall in my next give a few reasons for thinking this “ molten bomb” theory a mistake. I am, &c., .B. J. Westbeooke. P.S.—Since I wrote the above I have observed Mr Oliver’s letter. I thank him for his criticism, and shall, with your permission, have something to say to him in an early issue.
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