Imágenes de páginas
PDF
EPUB

feet.) At this place the following mean temperatures and epochs of maxima have been obtained:

[merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small]

For the greatest depth, the time required for the heat-wave to reach it is nearly six months, making the surface maximum temperature coincident with the minimum temperature of the stratum, 7.32 meters below.

In the case of Chicago, we may take the mean annual temperature of the atmosphere just above the surface=8°.17 C.; that of the surface of the soil 80.0 C. The increase for a depth of 12.2 meters for the lowest stratum of clay and to the rock-surface will be 0°.44 C., making 80.44 C. for this depth. The invariable temperature may be estimated about 9 or 10 C., and may be found at a depth between 30 and 40 meters, the variations being transmitted to a greater depth in rock than in clay. At the surface of the rock the variations of temperature will probably yet amount to 00.1, (according to the Edinburgh observations.) For a depth of 229 meters, corresponding to the depth from which the wellwater flows, the increase of heat should amount to 8°.2 C., according to the mean given above; hence the computed temperature 160.2 C. But the observed temperature is only 120.8 C., showing either a much slower rate of increase (1° C. in 48 meters) or a local deviation, probably due to infiltration of water from a higher level. Whether the lake-water, which is colder at the same depth than solid matter would be, can exert an influence by conduction, I do not know.

Prof. J. D. Everett, chairman of the committee of the British Association for the Advancement of Science, on the subject of underground temperature has published a number of valuable reports The following is an extract from a recent communication of his to the Belfast Natural History and Philosophical Society:

"The phenomena of underground temperature may conveniently be classed under two heads, according as attention is directed to the first forty or fifty feet, or to such depths as are attained in mines and artesian wells.

"The annual wave of temperature is propagated downward from the surface, at a rate which depends on the nature of the soil, and is on the average rather greater than a foot per week; while at the same time the amplitude (or magnitude) of the wave diminishes in a ratio also dependent on the soil, and amounting on the average to a halving of the amplitude for every five or six feet of descent.

"Supposing the soil to be uniform, the surface to be plane, and the propagation of heat to be effected solely by conduction, a simple harmonic variation of temperature at the surface (which we may call in popular language a simple wave of temperature) will be propagated downward with a uniform velocity, and with amplitude diminishing in geometrical progression. There will, moreover, be a definite relation between the ratio of this progression and the velocity of propagation, so that if the one is given the other can be computed. In fact, we shall have

[merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small]

where denotes 3.1416; c, thermal capacity per unit-volume; and, k, conductivity.*

"If the variation of temperature at the surface, instead of being simple harmonic, be any periodic variation whatever, it can be reduced by Fourier's method to the sum of a number of simple harmonic variations, and each of these variations will be propagated according to the above law, unaffected by the rest.

"As the square root of the number of days in the year is almost exactly 19, the above formula shows that the annual wave is propagated 19 times as fast as the diurnal wave, and that the falling off in amplitude is the same in one foot for the diurnal wave as in 19 feet for the annual wave. "Of the different simple harmonic components which make up the whole variation at the surface, those of longest period are propagated downward most quickly, and die away most slowly. For this reason,

*The numerical value of the co-efficient

[ocr errors]

as given in Professor Everett's Discussion of the Observations at the Greenwich Observatory, 1860, is as follows:

From the Greenwich observations..

From Calton Hill, trap-rock..........

From Experimental Garden, sand..

From Craigleith Quarry, sandstone

0.0918

0.1156

0.1098

0.0674

The diminution of indicates either a decrease in capacity for heat or an increase

in conductivity.

k

as well as from its greater original magnitude, the annual simple harmonic wave becomes more and more predominant as we descend, and the curve of temperature for the year approaches more and more nearly to the form of a simple harmonic curve, or curve of sines.

"Observations taken at three stations in or near Edinburgh, and at Greenwich Observatory, have been reduced in accordance with the above principles, the result being in every case to show a satisfactory agreement between theory and practice; and the values of the thermal coefficient thus obtained for these four stations, have furnished the basis k of the most reliable calculations yet made regarding the earth's age as a habitable globe. For the three Edinburgh stations the value of c (which is the product of specific heat by specific gravity) was also determined by laboratory experiments conducted by Regnault, and hence the conductivity, k, was found by computation.

"The following is a sample of the temperatures observed at Greenwich at the depths of 1 inch, 12.8 feet, and 25.6 feet. The warmest and coldest calendar months had the following mean temperatures:

[blocks in formation]

"The mean temperature at a depth of 10, 20, or 30 feet does not differ much from the mean temperature at the surface. A slight increase is, however, usually observable even at these small depths; and, when we penetrate to the depth of several hundred feet, we find the temperature higher by several degrees than the mean temperature of the surface. In fact, the deeper we go the higher is the temperature which we find. "Attempts were formerly made to explain away this phenomenon, the high temperatures observed in deep mines being ascribed to the pres ence of the men working in them, assisted in some cases by the slow combustion of pyrites; but the fact of a steady increase downward, at a rate which is not exactly uniform, but varies from about 1° Fahr. in 100 feet to 10 Fahr. in 40 feet, has now been placed beyond all question."

Theoretical investigations will be found in Fourier's Théorie analytique de la chaleur, Paris, 1822; in Poisson's Traité mathématique de la chaleur, Paris, 1835. See also various papers by Quetelet in the Mém. de l'Acad. roy. de Bruxelles; also, Piazzi Smyth in Astronomical Observations at the Royal Observatory at Edinburgh; Forbes and his own Observations, vol. xi, for 1849-254, and vol. xiii, for 1860-270. A fair statement of the subject is found in Schmid's Meteorology, Leipsic, 1860. See, also, J. D. Forbes's "Experiments on the Temperature of the Earth," in Trans. R. S. E., 1846; Sir W. Thomson "On the Reduction of Observations of Underground Temperature," in Trans. R. S. E., 1860; "On the Age of the Sun's Heat," in Macmillan's Magazine, March, 1862; "On

the Secular Cooling of the Earth," in Trans. R. S. E., 1862, (reprinted at the end of Thomson and Tait's "Treatise on Natural Philosophy;") "The Doctrine of Uniformity in Geology Briefly Refuted," in Proc. R. S. E., December, 1865; "On Geological Time," in Trans. Geol. Soc., Glasgow, vol. III, part I; "Of Geological Dynamics,” in Trans. Geol. Soc., Glasgow, vol. III, part II; J. D. Everett "On a Method of Reducing Observations of Underground Temperature," in Trans. R. S. E., 1860; "On the Mean Temperature of a Stratum of Soil," in Trans. R. S. E., 1862; "Reduction of the Observations of the Deep-sunk Thermometers," in Greenwich Observations, 1860; "Reports of Committee on Rate of Increase of Underground Temperature," in B. A. Reports, from 1868 onward.

ON A SERIES OF EARTHQUAKES IN NORTH CAROLINA, COMMENCING ON THE 10TH OF FEBRUARY, 1874.

BY PROFESSOR WARREN du Pré,

Of Wofford College, Spartanburgh, S. C.

The following is an extract from a letter of the 24th April, 1874, from Professor Du Pré to General Benjamin Alvord, U. S. Army :

"My visit to the mountains of North Carolina was undertaken to satisfy myself with respect to the numerous rumors which had reached us of the volcanic disturbances in that section of the country. I could spend but two days (19th and 20th March) in the investigation, but was quite diligent in collecting facts and in extending my explorations on horse and on foot so as to cover a distance of eighteen or twenty miles. I was soon convinced that the physical disturbances were real, but many of the rumors were false, and that the truth had been much exaggerated by the fears of the people. The explosive noises accompanying the shocks and the limited area of the disturbances are peculiarities worthy the attention of scientists, and demand a more thorough exploration. The inclosed is a hasty report of my trip, which I drew up to allay, if possible, the fears of the inhabitants of the district.

Stone Mountain, the site of the disturbances, like all the neighboring peaks, is composed chiefly of gneiss and granitic slates, and covered with a dense forest growth. In a direct line, it is about fourteen miles from Black Mountain, or "Mount Mitchell," the highest point in the United States east of the Rocky Mountains. It lies between Broad and Catawba Rivers, both of which point to Black Mountain, while on the northwest side of the Blue Ridge, the Swannanoah and Green Rivers, tributaries of the French Broad, have their sources near the Black Mountain. So many large rivers, on both sides of the Blue Ridge, heading up in this section, would indicate Black Mountain as the center of the volcanic force which lifted up these mountain-ridges. I expect to visit these mountains again in July, when I shall have more time to investigate this matter.*

[ocr errors]

Extracts from report above referred to, dated Spartanburgh, S. C., March 28, 1874.

On Wednesday, the 18th of March, in company with Rev. R. C. Oliver, editor of The Orphans' Friend, Mr. McKenn Johnstone, civil

* Professor Du Pré has been requested to communicate a report of this projected trip. Further data remain to be gleaned, as the phenomena appear to have continued, at least, up to April.

« AnteriorContinuar »