sustained by the stakes of an outer corridor, prevented the whole from going over. In the parlors to the mansion of this hacienda, things were thrown in all directions: lamps, chairs, books, fell in every possible line, almost inducing belief that the increasing resistance offered to the onward movement of the explosive agent by the rapidly approaching mountains, had converted rectilineal into gyratory motion. These objects fell in the several directions at different periods.

At the time of the shock, the proprietor was in the fields giving orders for the work of the day. Turning at the first rumble that reached his ear in the direction of the mansion, where his wife and children were, he put spurs to his horse, which had not yet become frightened. But an instant after, the poor brute suddenly stopped and spread out its feet, giving expression to the utmost terror by deep-breathed snorts and starting eyes, nor could any punishment make it move until the phenomenon had ceased. Apart from thoughts of his loved ones, this was a trying interval to my friend. Alone, and all nature convulsed! The earth heaved and trembled till foot-hold was not secure, its profoundly vaulted caverns pealing thunders stunning to the ears; the atmosphere was serene and balmy without a stirring breath, yet trees around were waving and bending half way to the very soil as in a storm; flocks of birds in rapid flight screamed their sympathy; and herds of cattle came tearing down the mountain sides, pursued by great boulders of granite, mid clouds of dust and sparks of fire.

Along the line of the road beyond the Angostura, there were scarcely any visible effects; and although the inhabitants of Rancagua say that the shock was extremely severe, there were only a few small crevices in the higher walls. If not exhausted to the northward of the gorge, the strength of the earth-storm had evidently passed to the westward of it, a supposition to which neither examination nor inquiry lent their support. The mountains had arrested the progress of the great earth-wave, and the re-action of its generating power was plainly exhibited on the alluvial strata of the deep terrestrial bay.

Travellers from as far south as Talca stated that the shock had been quite moderate at that city, and none had given it a thought beyond the Cachapual, except for its unusual length. Nearly all of them, however, as did those north of the Angostura, believed that its origin had been to the southward. Whilst at Rancagua, a violent rain-storm commenced on the morning of the 5th, preceded by excessive thunder and lightning. This was a widely extended storm, reaching from latitude 33° to latitude 40°.

As nothing further was to be gained in a southerly direction, I returned to Santiago, and two days afterwards crossed the axis of the earth-wave in the direction of Valparaiso, though without

obtaining many new facts to relate. The disturbance had certainly been greater at Curacavi and Casablanca than at the capital and port, much property having been so injured that it was necessary to tear it down. Repairs were out of the question, for the walls were no longer stable. Oue crack in the earth, west of Casablanca, at the surface, was still nearly three inches wide, and about two hundred yards long. Its general direction was WNW and ESE. The same fact was observed on the Almendral as had been remarked near the Angostura; objects were thrown from tables and shelves in every imaginable direction, as though each vibration was from a different quarter. No special agitation was observed at the surface of the sea, nor did any great wave follow to wash away prostrate buildings, of which some forty were level with the ground. One of the papers stated that a lead line thrown overboard at the time from the U. S. frigate Raritan, was so buried in the sand that it could only be extracted with great difficulty; but this, like many of the wonderful stories told of earthquakes, should probably be received "cum grano salis;" else we must conclude that the ships, being unable to heave them up, probably left their anchors in the bay when about to sail. There was no indication whatever that the shores of the bay had been raised either by the great shock or the multitude of smaller ones continuing throughout the succeeding fortnight. I examined the rocky shores closely during several tides, but could find no unprotected memento.

Mr. R. Budge, F.R.G.S., considers* the motion to have been westward, because water in basins, jugs, &c., spilt over the east side; clocks whose pendulums vibrated east and west stopped, while those beating north and south did not; walls standing east and west were cracked in every way-particularly lengthways, and vessels at sea felt it at an hour corresponding to the difference of longitude. He supposes the phenomenon to have been subject to instantaneous cessations, and says that it turned round things on their base instead of throwing them down at an angle of 20°, showing a circular motion for at least an instant. 1 shall have something to say presently respecting the two vessels which felt the shock at sea. He goes on to remark: "I have experienced at this place (Valparaiso) three ruinous earthquakes-that of 1822, which I passed in the house until the back fell, that of 1829, and the present. On the last occasion the barometer and thermometer indicated nothing, nor was there the least warning of any description; but, as invariably occurs after a heavy shock, we had, on the third day after, a shower of twelve hours' rain, for which I had already prepared, aware of its being the consequence, happen whatever season it may. I conceive also that I have felt less relaxed than before it. I cannot understand all

* Report of British Association, 1851.

these things, unless electricity be the agent; while the atmosphere must be affected in some way to shower down rain at seasons when, under ordinary circumstances, it does not fall. *** On that occasion (1822) the sea in the bay of Valparaiso retired considerably, and was several days in reaching its former level; while on this, no such thing was observed."

Only two vessels bound to Valparaiso felt the shock. One was forty miles to the southwest of the port, and the other a like distance to the northwest, and therefore they were some fifty-seven miles apart. Until he learned, after anchoring, that an earthquake had occurred on the morning of the 2nd, the master of the former was fully persuaded he had passed over a reef of rocks; the other felt no shock whatever, though at the time designated the crew had heard explosions like distant discharges of heavy artillery. San Antonio, near the mouth of the Maule, and Talcahuano, both experienced a tremor; Melipilla, between San Antonio and the capital, felt it severely. There was a violent shake at Quillota, also, and San Felipe de Aconcagua suffered some injury. Even the Copiapó papers mention a "temblor" on the morning of the 2nd; but nothing special was remarked, and it passed as one of those occurring almost daily.

Efforts to obtain reliable data for determining the velocity of earth-waves meet with little encouragement among those with whom "mañana" (to-morrow) is proverbial, and who have not yet learned that a few minutes are worthy of appreciation. Moreover, people generally are too much alarmed when the shock comes. Eternity occupies more of their thoughts than time; and had they self-possession to record the instants, probably no two time-keepers in the city agree within several minutes. Of the great shock one Talca paper says, "this morning a quarter before seven;" the other, "at twenty minutes past six in the morning." Even in Valparaiso, where government has placed a clock visible to nearly all the town, the papers differ two minutes, though the custom-house clock was stopped by the shock at 6h 42m. But here are the Santiago mean times at which the greatest shock was felt at each place, with its bearing and distance from the capitol.

For days-it may be said weeks-after, the whole district of country disturbed by the principal shocks was visited by tremors. At Santiago the times of four were noted on the 3d; only one on the 4th; two or three on the 5th; and so on up to the 20th; indeed, for several months their occurrence was more frequent than during the same period of the preceding year. Having passed from the afternoon of the 6th at Aguila, the hacienda of a friend within the deep bay of the mountains, there were opportunities to experience some of them in the open fields.

ART. XLIV.-Supplementary Note to the article on the Theory which attributes the Zodiacal Light to a Nebulous Ring surrounding the Earth;* by F. A. P. BARNARD, LL.D., Professor of Mathematics and Astronomy in the University of Mississippi.

Prof. DANA,-Will you allow me space for a few words supplementary to the article on the zodiacal light, published in the March number of the Journal of Science. In that article the geographical limits of visibility of the cusps of a ring encircling the earth, lying in the plane of the ecliptic, illuminated by the sun and interrupted by the earth's shadow, are assigned for the moments when the sun is eighteen degrees below the eastern or western horizon. It is true, however, that there are certain limits of distance from the earth's centre, between which, if such a ring be situated, a certain portion of the part of it illuminated may be visible, under the circumstances supposed, as an illuminated arch, though the cusp may not be above the horizon. In order to determine these limits, if we

resume the consideration of fig. 5 in the article referred to, where AOB is the earth, HZR the imaginary spherical surface of " which the ring is a great circle, H'R' the horizontal small circle of this sphere passing

R

through the place of observation O, S the pole of the limiting circle (i. e. the circle of the shadow), SQ its arc-radius, and Q the cusp of the luminosity, then when Q is on the horizon, the ring may touch the horizontal circle H'R', or it may rise above it either on the illuminated or on the obscured side of Q. In the former case, a luminous arch will be visible to the observer at O. The tangency or the intersection which takes place at Q will be determined by the value of the angle at that point; contact occurring when this angle is ninety degrees, and the intersection favoring the visibility of the light, when the angle is greater than ninety.

*This volume, p. 217.

Put O's depression =, ZQ=g, SQ=g', ZQS=Q. Then, in the triangle ZQS

sin cos g' cos g+sin g' sin g cos Q.

But, by the hypothesis, e+g=90°. Consequently cos '=sin e; and sin g=cos g.

Whence,

Sin d=sin e cos ? (1+cos Q) = sin 2o (1+cos Q).

When 2 sin d=sin 2 g, cos Q=0, and the ring touches the horizon. When 2 sin &> sin 2 g, cos Q is positive, and the obscured part of the ring rises above the horizon. When 2 siu &< sin 2g, cos Q is negative, and the light is visible to the observer at O.

It is evident that, & being constant, this phenomenon will be more remarkable as sin 2 g is greater. Putting, therefore, sin 29=1, we have o=45°; whence it is evident that such a ring as we have been considering would be most conspicuous as a luminous arch, after the setting of the cusp, provided it were placed at a distance from the earth's centre =3956√2, or 1640 miles above the surface.

=

If, while we make sin 2 g=1, we make also 2 sin d=1, the ring will touch the horizon, and we shall have sin d= sin 30°. That is to say, the maximum depression of the sun at which the phenomenon can occur, is thirty degrees.

୧

If we take the sun's depression = 18, as in the article referred to, then tangency will occur, when sin 2 g 2 sin 18°: which will give the value of 9=19° 5′, or 70° 55'. If any value be assumed for between these limits, the phenomenon will be observable; but for any value beyond them it will not be observable at this depression of the sun. But the distance of the ring from the earth's centre corresponding to any value of e, is expressed by the formula,

in which D represents this distance, and R the earth's radius. Whence the limits of distance which will make the light visible. at the close of twilight or at the commencement of the dawn, though the extremity may be beneath the horizon, are 4186 miles for the lesser, and 12,100 miles for the greater, from the centre of the earth.

The inclination of the ring to the horizon when it just touches the circle H'R' is equal to 90°-ZR'g'. The geographical limits within which the light of a ring situated at any distance from the earth between the limits just determined, ought never to be absent in some form and during some portion of the night,