that the mean specific gravity of the earth would be about five times that of water. The only objection to this admirable experiment is, that the form of the country near the mountain is very irregular, and it is difficult to say how much of the 12 seconds is or is not really due to Schehallien.

The second class is what may be called a cabinet experiment, possessing the advantage of being extremely manageable, and the disadvantage of being exceedingly delicate, and liable to derangement by forces so trifling that they could with difficulty be avoided. Two small balls upon a light horizontal rod were suspended by a wire, or two wires, forming a torsion balance, and two large leaden balls were brought near to attract the small balls from the quiescent position. We could make a calculation of how far the great balls would attract the little ones, if they were as dense as the general mass of the earth; and comparing this with the distance to which the leaden balls really do attract them, we find the proportion of the density of the earth to the density of lead. The peculiar difficulty and doubt of the results in this experiment depend on the liability to disturbances from other causes than the attraction of the leaden balls, especially the currents of air produced by the approach of bodies of a different temperature; and after all the cautions of Cavendish, Reich, and Bailey, in their successive attempts, it seems not impossible that the phenomena observed may have been produced in part by the temperature of the great balls as well as their attraction.

These considerations induced Mr. Airy, in 1826, to contemplate a third class of experiments, namely, the determination of the difference of gravity at the top and the bottom of a deep mine, by pendulum experiments. Supposing the difference of gravity found, its application to the determination of density (in the simplest case) was thus explained. Conceive a spheroid concentric with the external spheroid of the earth to pass through the lower station in the mine. It is easily shown that the attraction of the shell included between these produces no effect whatever at the lower station, but produces the same effect at the upper station as if all its matter were collected at the earth's centre. Therefore, at the lower station we have the attraction of the interior mass only: at the upper station we have the attraction of the interior mass (though at a greater distance from the attracted pendulum) and also the attraction of the shell. It is plain that by making the proportion of these theoretical attractions equal to the proportion actually observed by means of the pendulum, we have the requisite elements for finding the proportion of the shell's attraction to the internal mass's attraction, and therefore the proportion of the matter in the shell to the matter in the internal mass; from which the proportion of density is at once found. Moreover, it appeared probable, upon estimating the SECOND SERIES, Vol. XXI, No. 63.—May, 1856.

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errors to which observations are liable, that the resulting error in the density, in this form of experiment, would be less than in

the others.

Accordingly, in 1826, the speaker, with the assistance of his friend Mr. Whewell (now Dr. Whewell), undertook a series of experiments at the depth of nearly 1200 feet, in the Dolcoath mine, near Camborne, in Cornwall. The comparison of the upper and lower clocks (to which further allusion will be made) was found to be the most serious difficulty. The personal labor was also very great. They had, however, made a certain progress when, on raising a part of the instruments, the straw packing took fire-(the origin of the fire is still unknown)—and partly by burning, aud partly by falling, the instruments were nearly destroyed.

In 1828 the same party, with the assistance of Mr. Sheepshanks and other friends, repeated the experiment in the same place. After mastering several difficulties, they were stopped by a slip of the solid rock of the mine, which deranged the pumps and finally flooded the lower station.

The matter rested for nearly twenty-six years, the principal progress in the subjects related to it being the correction to the computation of "buoyancy" of the pendulum, determined by Colonel Sabine's experiments. But in the spring of 1854, the manipulation of galvanic signals had become familiar to the Astronomer Royal, and the assistants of the Greenwich Observatory, and it soon occurred to him that one of the most annoying difficulties in the former experiment might be considered as being practically overcome, inasmuch as the upper and lower clocks could be compared by simultaneous galvanic signals. Inquiries, made in the summer, induced him to fix on the Harton colliery near South Shields, where a reputed depth of 1260 feet could be obtained; and as soon as this selection was known, every possible facility and assistance were given by the owners of the mine. Arrangements were made for preparing an expedition on a scale sufficient to overcome all anticipated difficulties. A considerable part of the expense was met by a grant from the Board of Admiralty. The Electric Telegraph Company, with great liberality, contributed (unsolicited) the skill and labor required in the galvanic mountings. The principal instruments were lent by the Royal Society. Two observers were furnished by the Royal Observatory, one by the Durham Observatory, one by the Oxford Observatory, one by the Cambridge Observatory, and one by the private observatory of Red Hill (Mr. Carrington's). Mr. Dunkin, of the Royal Observatory, had the immediate superintendence of the observatious.

The two stations selected were exactly in the same vertical, excellently walled, floored, and ceiled; the lower station in par

ticular, was a most comfortable room or rather suite of rooms. Every care was taken for solidity of foundation and steadiness of temperature. In each (the upper and the lower) was mounted an invariable brass pendulum, vibrating by means of a steel knife edge upon plates of agate, carried by a very firm iron stand. Close behind it, upon an independent stand, was a clock, carrying upon the bob of its pendulum an illuminated disk, of diameter nearly equal to the breadth of the tail of the invariable rendulum; and between the two pendulums was a chink or opening of two plates of metal, which admitted of adjustment, and was opened very nearly to the same breadth as the disk. To view these a telescope was fixed in a wall, and the observer was seated in another room. When the invariable pendulum and the clock pendulum pass the central points of vibration at the same instant, the invariable pendulum hides the illuminated disk as it passes the chink, and it is not seen at all. At other times it is seen in passing the chink. The observation, then, of this disappearance determines a coincidence with great precision. Suppose the next coincidence occurs after 400 seconds. Then the invariable pendulum (swinging more slowly), has lost exactly two swings upon the clock pendulum, or the proportion of its swings to those of the clock pendulum is 398: 400. If an error of a second has been committed, the proportion is only altered to 397: 399, which differs by an almost insignificant quantity. Thus the observation, in itself extremely rude, gives results of very great accuracy. As the proportion of invariable-pendulum-swings to clockpendulum-swings is thus found, and as the clock-pendulumswings in any required time are counted by the clock dial, the corresponding number of invariable-pendulum-swings is at once found. Corrections are then required for the expansion of the metal (depending on the thermometer-reading), for the arc of vibration, and for the buoyancy in air (depending on the barometerreading).

But when the corrected proportion of upper-invariable-pendulum-swings to upper-clock-pendulum-swings is found, and the proportion of lower-invariable-pendulum-swings to lower clockpendulum-swings is found, there is yet another thing required :— namely, the proportion of upper-clock-pendulum-swings to lowerclock-pendulum-swings in the same time; or, in other words, the proportion of the clock rates. It was for this that the galvanic signals were required. A galvanometer was attached to each clock, and an apparatus was provided in a small auxiliary clock, which completed a circuit at every fifteen seconds nearly. The wire of this circuit, passing from a small battery through the auxiliary clock, then went through the upper galvanometer, then passed down the shaft of the mine to the lower galvanometer, and then returned to the battery. At each galvanometer there

was a small apparatus for breaking circuit. At times previously arranged, the circuit was completed by this apparatus at both stations, and then it was the duty of the observers at both stations to note the clock times of the same signals; and these evidently give comparisons of the clocks, and therefore give the means of comparing their rates. Thus (by steps previously explained), the number of swings made by the upper pendulum is compared with the number of swings made in the same time by the lower pendulum.

Still the result is not complete, because it may be influenced by the peculiarities of each pendulum. In order to overcome these, after pendulum A had been used above and pendulum B below, they were reversed; pendulum B being observed above and A below; and this, theoretically completes the operation. But in order to insure that the pendulum received no injury in the interchange, it is desirable again to repeat the experiments with A above and B below, and again with B above and A below.

In this manner the pendulums were observed with 104 hours of incessant observations, simultaneous at both stations, A above and B below; then with 104 hours, B above and A below; then with 60 hours, A above and B below; then with 60 hours, B adove and A below. And 2454 effective signals were observed at each station.

The result is, that the pendulums suffered no injury in their changes; and that the acceleration of the pendulum on being carried down 1260 feet is 2 seconds per day, or that gravity is increased by part.

It does not appear likely that this determination can be sensibly in error. The circumstances of experiment were, in all respects, extremely favorable; the only element of constant error seems to be that (in consequence of the advanced season of the year), the upper station was cooler by 7° than the lower station, and the temperature-reductions are therefore liable to any uncertainty which may remain on the correction for 7°. The reductions employed were those deduced by Sabine from direct experiment, and their uncertainty must be very small.

If a calculation of the earth's mean density were based upon the determination just given, using the simple theory to which allusion is made above, it would be found to be between six times and seven times the density of water. But it is necessary yet to take into account the deficiency of matter in the valley of the Tyne, in the hollow of Jarrow Slake, and on the seacoast. It is also necessary to obtain more precise determinations of the specific gravities of the rocks about Harton colliery than have yet been procured. Measures are in progress for supplying all these deficiencies. It seems probable that the resulting number for the earth's density will probably be diminished by these more accurate estimations.

ART. XXXVIII.-On the rate of Evaporation on the Tulare Lakes of California; By WM. P. BLAKE, Washington, D. C.

Read before the National Institute, Washington, D. C., March 4th, 1856.

THE Tulare plains of California wear a most desert-like and barren aspect during the summer and autumn. Treeless and without green vegetation, the surface becomes parched by the rays of an unclouded sun, and gives unobstructed passage to steady currents of air which pass inland from the ocean towards the Sierra Nevada. These winds, after passing the ranges of the Coast Mountains and becoming partially desiccated in their transit, impinge upon and traverse the plain, and reach the foot-hills of its eastern margin with a high temperature and apparently little moisture.

In the month of August, 1853, while with the U. S. Pacific R. R. survey, commanded by Lieut. R. S. Williamson, we encamped on the banks of Ocoya Creek, among the foot-hills of the Sierra, and every day felt the hot wind blowing inland toward the mountains. The parching effect produced by these winds, and the fact that after leaving the Coast Mountains they sweep over the broad and shallow Tulare lakes, induced me to desire to ascertain the rapidity of evaporation of water when fairly exposed to their action, and if possible to arrive at an approximate estimate of the amount of water removed daily from these lakes. For this purpose, I made the experiment of exposing water in a pan to the action of the wind, and noting the amount removed each day.

The valley of Ocoya Creek, in which our camp was located, has been formed by the erosion of the creek in a plateau of soft tertiary strata. It is thus bounded on each side by hills of horizontal stratification, and these were from 300 to 900 feet in height. They were perfectly barren and parched, and the only green vegetation visible was confined to the immediate banks of the creek. The altitude of the camp was 738 feet above meantide, and its distance from the open plain of the Tulares, two miles, from the lakes 25 miles, and from the sea in a direct east and west line 120 miles. The width of the valley was about one quarter of a mile, and its direction was nearly east and west, so that the breeze from the plain followed its course without deflection.

The wind usually blew gently from the mountains during the night and early in the morning, but after the sun had risen, and about 10 o'clock its direction was reversed and it blew steadily and often strongly from the west or northwest until sunset, when it generally ceased.