ART. XVIII.-Observations of Tides at Tahiti, made for the U. S. Coast Survey, under the direction of Captain JOHN RODGERS, U. S. N. Communicated by Prof. A. D. BACHE, Supt. U. S. Coast Survey.

THE tides at Tahiti have long been known to exhibit the peculiarity of occurring nearly at the same hour of every day, indicating an almost total elimination of the lunar tide.

As far as we know, the tides of no other part of the globe present this extraordinary feature. No explanation fully satis factory has yet been proposed for this phenomenon.

From their bearing on the laws regulating the tides on the North American coast of the Pacific Ocean, accurate tidal observations in central parts of that ocean are a great desideratum, and advantage was therefore taken by the Superintendent of the Coast Survey, of the surveying expedition under the command of Captain John Rodgers, U. S. N., by furnishing him with one of Saxton's self-registering tide-gauges, with the request to set it up at some suitable point.

The town of Papeete, on the island of Tahiti, was selected by Captain Rodgers, and the tide-gauge left there under the charge of an intelligent French soldier. The observations began on the 27th of April, 1858, but up to June 2d were so frequently interrupted as to be of little use. After that date, they are nearly complete to October 12th, subsequent to which time no observations have been received.

On the accompanying diagram the mean local time of each high-water has been plotted in such a manner that the abscissæ represent the days of the month, the ordinates the hours of the day. An inspection of the diagram will show that during part of the month of June the tide appears to have followed the general rule, occurring later every day, so that there was a highwater successively at all the hours of the twenty-four. In July, however, the case was quite different. The high-waters occurred at a later hour on successive days, but only until they had reached three or four hours, and, in one case, five, after noon or midnight, when they came back abruptly to the neighborhood of 12, to follow again a similar cycle. This type they preserved to October 12th, when the observations ceased, only the range and the abruptness of the return to an earlier hour becoming lessened.

It will also be remarked that the midnight tides diverge less from that hour than the midday tides from noon. The times of low-water have, as may be expected, similar relations to 6 A. M. and 6 P. M.

Taking the mean of all the observations we find 59 minutes past noon, and 53 minutes past midnight, for the average time of high-water, and 45 minutes past six for the average time of lowwater, both morning and night.

The smaller diagram shows the variations from that mean, arranged according to the moon's transit. The curve exhibited might be called the half-monthly inequality in the soli-tidal interval, in analogy to the curve of half-monthly inequality of the luni-tidal interval of ordinary tides.

With regard to the heights, the statements received are not quite precise as to the scale used in the self-registering tide-gauge, and some uncertainty arises from the zero-point having been repeatedly altered, owing to the necessity of using the same paper on the gauge several times. On the best supposition that can be made we find the mean rise and fall to be 0.87 ft. The observations made at the same port, for a few days, by Capt. Sir Edward Belcher, R. N., in 1840 (Phil. Trans., 1843) gave a mean rise and fall of only 0.56 ft. The diurnal inequality is well marked, and the half-monthly inequality regular and normal.

If we examine the variations of the intervals near the time of the summer solstice, we shall find that on the 3d of June or a little after, when the moon's upper transit comes at noon, the intervals for midday tides have their mean value. Then the interval increases rapidly every day as the moon's transits come later, until they come near the middle of the afternoon, when there is a sudden change of 6h or 8h in the course of 2 or 3 days, during which time, the tides are so small that their times and intervals cannot be well determined. The high-waters then seem to pass under the influence of the moon's inferior transits, and the intervals are reduced to their minimum values, or become negative. The intervals then increase rapidly again for a considerable part of half a lunation, until they pass again under the action of, and finally under the control of, the moon's superior transits. The same law will be observed to prevail throughout the period of observation, but the inequality rapidly decreases in amount as we depart from the solstice. Similar observations apply mutatis mutandis to the midnight tides.

The range of the tides seems to be considerably less near the solstice, than they are near the equinox.

There seem to have been some notable changes in the mean level of the ocean in the month of July.

Hopes are entertained of obtaining more observations, and a supply of paper was forwarded to the U. S. Consul some years. ago, but no answer has ever been received. It has been thought advisable therefore to publish the results as far as they have been obtained.

AM. JOUR. SCI.-SECOND SERIES, VOL. XLI, No. 122.-MARCH, 1866.

IN

ART. XIX.-On Prairies; by A. FENDLER.'

In my botanical rambles I have seen Prairies, Llanos and Savannahs of different magnitudes and in various stages of progress; and, as I am assured that my suggestions on this subject, although they may not fully coincide with your views, would be received in a kind spirit, I beg to make the following re

marks.

The prevalence of moisture, although generally is not always connected directly with a prevalence of forest. For four years I lived on the range of mountains that stretches along the northern coast of Venezuela in lat. 10° N., at an elevation of 6,500 feet above the sea, in the midst of a vast forest. The northern slope of this chain of mountains, from its very crest (7,000–8,000 ft.) down to the margin of the sea, is covered by an immense primeval forest, except in a very few insignificant spots, near some of the seaports, where man has interfered with it. In this mountainous region, from the middle of April to the first of January, hardly a day passes without rain. It is a region enveloped in mist and clouds during most of this time, alternately drizzling, dripping and pouring down, where the temperature rarely ever rises as high as 70° or falls as low as 38°. It is a true "Fern-region," where tree-ferns may be found from 30 to 40 feet high. It is certainly one of the most rainy, moist and humid regions outside of the great equatorial rain-belt. Although the northern or sea-side of this mountain-range is in its whole length covered by forest, savannahs stretch in many places from the very crest down the opposite or southern side, which is constantly exposed to southern or southeastern winds, driving the clouds and mist against it and up along its surface away over the crest. In some localities of the windward side forests alternate with savannahs, but in other localities on the same side, and exposed to the same wet weather, large tracts are entirely bare of forest. The forests, however, do not gain on the savannahs, but from time to time the savannahs gain on the forests by a very simple agent, namely, that of fire.

Fire I consider by far the most powerful and the principal agency that gave prairies and savannahs their existence, extending them in the course of time and still busy in extending them. In the prairies of Illinois, Missouri and Texas as well as on the great plains between Missouri and the Rocky mountains, on the Ilanos of Venezuela and the high savannahs of the mountainous district of the same country, I have seen the same forest-destroyer at work. In the region last named I had the rare opportunity of observing not only the gradual extension of the savannahs,

From a letter to Professor Dana, dated Cambridge, Mass., Dec. 12, 1865.

but also the beginning and the different stages of the conversion of an almost impenetrable primeval forest into a savannah.

I cite the following facts, observed by myself at Colonia Tovar ín Venezuela, from an account in the Smithsonian Report for 1857, pages 186-188.

"On the 5th of January I made a botanical excursion to one of the highest mountains of this region, about twelve miles to the east of the colony. The mountain, according to my estimation, may be about 7,800 feet above the level of the sea, and is a kind of central point or knot, from which several rivers, flowing in different directions, take their origin. This mountain is covered by a dense forest, with the exception of a level spot of about half a mile in length and a quarter of a mile in width, which forms a kind of shallow basin, only sparingly covered by a thin coat of short grass and other small plants. * * *

In this excursion I had also an opportunity to form some idea of the vast extent of destruction which was carried into the mountain forest last February by a lucifer match and a thoughtless boy. Over whole tracts of this primeval forest the trees lie dead, one over the other, as if uprooted by a whirlwind, scarcely showing any marks of fire on their trunks. I was struck more than ever with the easy manner in which fire can destroy these dense and humid forests, which, by their shade, preserve a cool and moist atmosphere, and thereby cause the vapors of the adjacent strata of air to condense into clouds, that rest upon them, with little intermission, during nine months in the year. In these high regions the temperature is so low and equable that the vegetable matter which is gathered on the ground between the trees is decomposed very, incompletely and very slowly. It forms a stratum of loose half-decomposed matter, in some places two to three feet thick, which, in the rainy season, like an immense layer of sponge filled with water, feeds and supplies the rivulets and rivers gradually. In the midst of the dry season this layer becomes sometimes dry enough to burn, when kindled, with but little flame, and more like tinder, spreading in all directions.

In this way the fire extends until met by a river or a road, or some other obstacle. The subsoil which underlies the spongy stratum on these mountains is also very shallow and resting on hard rocks. The roots of the trees, therefore, do not go down very deep, but extend more in a horizontal direction. When the spongy layer, with the smaller roots, are burnt, the trees lose their hold entirely and fall, one over the other, in all directions. They die less from being burnt than from being uprooted. Many different kinds of tall reeds soon take the place of the trees. In a few years these reeds exclude everything else. The fertile mould that may perhaps have escaped destruction by fire is by and by carried down the declivities by the frequent rains. The region, no longer shaded by high trees, becomes dry. Subsequent conflagrations of adjacent savannahs, which are intentionally set on fire to procure a new growth of young grass, take hold of the reeds of the ruined forest, until, by the repeated attacks of these fires, the roots of the reeds can stand it no longer, and the smaller grasses, interspersed with a few other plants, take their places.