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arch, from all parts of which numerous streamers darted upward toward the zenith.

Meanwhile, the zodiacal cone rose obliquely along the ecliptic, farther west. It was a much less conspicuous object than it had been two months before, when twilight was shorter. It was in no way altered from its usual appearance at this season, by the presence of the aurora. The southern edge of the cone was, as usual, better defined and more vertical than the northern edge; while the axis of greatest brightness, corresponding closely with the ecliptic, lay south of the axis of symmetry. Its base was widened out by atmospheric diffraction, and its apex merged into the zodiacal band. It was of about the same pale color as the aurora, having lost the warm color which can be noticed at its maximum of brightness. Unlike the flickering streamer of the aurora, it remained perfectly steady, and showed no fluctuations. of brightness. It was bright until about nine o'clock, and then, as its lower portion sunk below the horizon, gradually became more and more faint, until it could be no longer recognized.

The aurora went through a number of changes in detail, although maintaining the same general appearance for more than an hour. An arch, entirely disconnected with the lower arch, would sometimes form completely in a few seconds. This upper arch was composed of a number of very short rays placed laterally one along side of the other; and the arch extended by the addition of more short rays. Sometimes small clouds of light, not unlike cirrus clouds, formed near the horizon upon the longer rays, without interrupting them. Once an arch composed of short rays, each of which was some 3°-4°, and every fourth or fifth of which was longer and brighter than the others, formed suddenly and simultaneously across the sky, stretching from the zodiacal cone, against which one extremity apparently rested, to a point in Cepheus. New streamers were continually added to it in the east, and the short streamers composing it apparently drifted very slowly westward. At 9.27, this arch disappeared entirely in the course of a few seconds. At 9.34, it was again formed, stretching from 3 Taurus to Cepheus, and consisting as before, of short

rays.

At the formation of each arch, or when a new streamer is shot forth, the nebulous mass of light near the horizon would greatly

1 Note on the Zodiacal Light, by II. C. Lewis. Amer. Jour. Science, December, 1880.

brighten, as though it were a cloud illuminated by flashes of light. The streamers were nearly parallel to one another, their centre of divergence lying below the horizon. They were quite straight, and, unlike clouds, sharply defined on their edges. Often a streamer seemed best defined on the edge nearest the centre of the aurora. At one time, it was thought that a number of the streamers on the eastern half of the aurora were bent over at their upper portions toward the east, as though blown by a wind. The streamers were brighter and higher at the extreme east and west of the aurora, than at its central northern portion. The color of the streamers and of the bank of light below them was pale greenish-white. At first, a few short streamers of a warmer tint were occasionally seen.

At 9.33, the zodiacal cone being now low down towards the horizon, and pointing towards Præsepe, a very remarkable auroral streamer formed low down in the west. It was a bright,

short streamer, probably 2° or more in width, which pointed very slightly west of 0 Gemini. It started from near Aldebaran, now below the horizon, passed between and Tauri, and reached Auriga between z and 0. It was thus close to the edge of the zodiacal cone.

This streamer remained in the same position for nearly an hour. It was remarkably steady, and was not unlike a pale grayish-green comet's tail. For the first ten minutes, it was without any fluctuations. The arch died away, but this remained, until at 9.43, it too nearly disappeared, leaving only a trace of its position. Before ten o'clock, it was again bright. At 10.03, the rest of the aurora was dim, and no other streamers appeared. It was now very conspicuous, and remained so until nearly half-past ten, when it finally died away,—a faint haze on the northern horizon alone remaining. All this time, as seen from the position of this streamer back of certain trees and its angle with the horizon, it had maintained precisely the same position with reference to the earth.

Meanwhile, the heavens had revolved past it, the stars first noticed had moved beyond it, and the zodiacal cone had set. The streamer had remained like a great pointer fixed to the earth, marking its motion. While the zodiacal cone gradually sank below the horizon, the auroral streamer had retained its position,a circumstance well illustrating the cosmical character of the former and the terrestrial character of the latter.

Another fact pointing to the same conclusion was found in the difference of their spectra. In the early part of the evening, while both the zodiacal cone and the aurora were at their brightest, an opportunity was taken to compare their spectra. The observations were made with Eaton's direct vision spectroscope.

The zodiacal cone gave a faint, short, continuous spectrum, brightest near its least refrangible end. It was most sharply terminated at the same end, fading off gradually toward the more refrangible end. It was very faint throughout, and could be seen only through a wide slit.

The aurora gave a spectrum much longer than that of the zodiacal cone, though of about the same pale greenish color. At or close to the less refrangible end was a sharp, bright line of a grayish-green color. The spectrum from this line toward the more refrangible end was continuous and gradually diminishing in brightness, except that a very faint brightening near the blue showed that a brighter aurora might have given a line at that place. The green line of the aurora could be seen with a very much narrower slit than that required to see the spectrum of the zodiacal cone. If the spectra of the aurora and the zodiacal cone could be superposed, it would be found that the green line of the former is close to the more refrangible end of the spectrum of the latter.

The spectra show that while the light of the zodiacal cone is polychromatic, that of the aurora is nearly monochromatic, or, as it might be expressed, oligo-chromatic. The former is such as would be given by sunlight reflected on diffuse matter in space; the latter might be given by an electrical discharge through a gas.

Later in the evening, the faint zodiacal band was observed stretching completely across the sky along the ecliptic, while a round patch of light on the boundary between Virgo and Libra, lying apparently about 3° above the ecliptic, is recognized as the Gegenschein.

It is perhaps worth noting that no change of consequence in the weather or temperature took place during the two or three days preceding and following this aurora.

EXPERIMENTS ON THE STRENGTH OF YELLOW PINE. BY PROF. R. H. THURSTON, of Hoboken, N. J.

[ABSTRACT.]

In a paper read at the Saratoga meeting of the American Association for the Advancement of Science the writer presented the result of a series of experiments on the strength of timber, in which were given several unusual figures.1

To determine how far these results were due to peculiarities of the selected samples supplied from the Navy Yard, and to determine to what extent size affects the resistance, a more extended series of transverse tests were made, and the results of experiments upon yellow pine, of the ordinary market qualities and of various dimensions, are now presented below, as determined in the Mechanical Laboratory of the Stevens Institute of Technology.

In the paper referred to, the modulus of elasticity was given for yellow pine as an extraordinarily high figure. It will be observed that the best wood here described gives also very high values of E, and a comparison of the pieces of the first with these test specimens shows the selected Navy Yard specimen to have been of better material than either of the latter.

3

in

Samples marked F1, F2, Fa, were from the same plank-a piece of yellow pine cut in Georgia, April, 1879, and tested after several months of seasoning, when it had become thoroughly dry. The three specimens were considered good material. F, was not straight-grained and broke obliquely, giving a much lower modulus of elasticity, as well as of mixture, than its companion specimens. Samples B1, B2, B3, were cut from a stick ten inches square section, which had been lying under cover, seasoning, nine or ten years. Numbers 1 to 12, inclusive, were small sticks sawn out of the middle of a plank, originally four inches thick, one foot wide, and twenty-four feet long. A stick was first cut three inches square and twenty-four feet long, which was then cut into strips of varying smaller dimensions. The wood was selected from lumber-yard stock, and was considered to be fairly representative of average timber. It was cut in Florida, in October, 1879, and reached Hoboken in January, 1880.

Among these were several due to printers' errors, which, owing to the illness of the writer, were not corrected in proof. The tenacity of yellow pine, for example, was made 2070.2 when it should have been given at 20,702,

Specimens 1 to 8, inclusive, were too green for use in construction; Nos. 9, 10 and 11 were kiln-dried 56 hours, at a temperature of 130° Fahr., No. 12 was dried 12 hours, at 210° Fahr., at which temperature the pitch exuded from the wood quite freely. Still another specimen, not here recorded, was heated 13 hours at 420° F., and was somewhat charred. Under test it gave a modulus of rupture, R 9000.

In the table containing a résumé of results is also given the figures obtained in the earlier experiments on selected Navy Yard material, marked J and those of a sample, K, tested by another observer.2

Specimens A, A,, were picked up in a workshop, and used simply to determine specific gravity; they were probably not the best southern long-leaved pine, such as it was intended that the samples tested should be.

Van Nostrand's Magazine, Feb., 1880, p. 166.

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