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waves of the sea, may be considered as depending upon the following elements:

1st. The large basin between Sandy Hook and Staten Island, including Raritan Bay, which furnishes more than one-half of the whole ebbscour;

2d. What is called the Upper Bay, including the Jersey flats and Newark Bay;

3d. The North River, perhaps as far as Dobbs' Ferry, maintaining the head of the ebb-current, although not directly taking part in the outflow; and,

4th. A portion of the sound tide, which flows in through Hell Gate. The proportion of the three first divisions in producing the depth of channel may be approximately estimated by a comparison of the areas and distances from the bar. In order to maintain the depth which we now have, it is important that the area of the tidal basin should not be encroached upon. In proportion as that is diminished the depth of the channels will decrease.

The flats, just bare at low water, but covered at high tide, form as important a part as any other portion, for it is obvious that it is only the volume of water contained between the planes of low and high water-the "tide-prism "-that does the work in scouring the channels. The water on the flats is especially useful by retarding the outflow, thus allowing a greater difference of level to be reached between the basin and the ocean.

When we yield to the demands of commerce any portion of the tidal territory to be used for its wharves and docks, we must do so with full cognizance of the sacrifice we are about to make in the depth of water over the bar; and in order to form any well-founded judgment in regard to the effect of such encroachments, it is necessary to be in possession of the fullest knowledge of all the physical facts involved in the problem, and no measure of encroachment should be determined upon except in pursuance of the advice of scientific experts.

A proposition frequently mooted by men of enterprise, and resisted by those interested in the welfare of the city of New York, is the occupation of the Jersey flats from Paulus Hook to Robbins Reef for docks and wharves. Without expressing any opinion as to the relative value of the gain of accommodation for shipping and the loss of depth in the channel, I venture to say that the withdrawal of that area from the domain of the tide would occasion a loss of not less than one foot in the depth of the bar off Sandy Hook, and certainly not more than two feet.

The part which the fourth division in our classification of the basin of New York, that of the East River and Hell Gate passage, plays in the outflow of the ebb-tide through the Sandy Hook channels, depends less upon the area involved than upon the difference in point of time and height of tide in Hell Gate, already adverted to. The westerly

current, usually called the ebb-stream, since it falls in with the ebbstream of New York Harbor, taking place when the sound-tide is highest, starts from a level of three and a half feet higher than the easterly, and thus a much larger amount of water flows out through the Sandy Hook channels than through the narrows at Throg's Neck. It is apparent, then, that this portion of the ebb-stream, re-enforcing as it does the ebb-stream of the harbor proper at the most favorable times, performs a most important part in maintaining the channels through the Sandy Hook bar. It may be estimated that the closing of Hell Gate would cause the loss of certainly not less than three feet in the depth of those channels.

From what has been said with regard to the meeting of the tides in Hell Gate, it will be seen that the violent currents experienced in that locality are due to causes beyond our coutrol. The dangers to naviga tion arising from these currents, however, by their setting vessels upon the rocks and reefs, may, in a great measure, be done away with by the removal of the obstructions, in which work considerable progress has already been made. The removal of the reef at Hallett's Point, the work upon which is now in progress, will doubtless, in a great degree, do away with the eddies and under-currents produced by the sharp turn which the channel now takes at that point. It is not improbable that the successful removal of those obstructions will yet cause the sound entrance to be used in preference to the other by the fleets plying between European ports and the great commercial metropolis of America.

NOTE.—The reader who wishes to enter upon the mathematical treatment of the subject of tides is referred to Airy's treatise on tides and waves, and to the memoirs of Whewell and Lubbock, in the Philosophical Transactions of the Royal Society; and for investigations of the laws of the tides on our own coasts, to the papers on that subject by Bache and others in the annual reports of the Coast Survey. Among the latter, the lecturer is particularly indebte to the "Report on the tides and currents of Hell Gate," by Henry Mitchell, 1867, in which the complicated problem of the tidal circulation of New York Harbor is treated with great ability and success.

OBSERVATIONS UPON THE ELECTRICITY OF THE ATMOSPHERE AND THE AURORA BOREALIS, MADE DURING THE SWEDISH EXPEDITION OF 1868 TO THE NORTH POLE.

By Prof. SELIM LEMSTRÖM, of the University of Helsingfors, Finland.

There can be no savant now living who is not convinced that polar light is a phenomenon due to electric action in the upper regions of the atmosphere. Of the two theories which have been advanced to explain the phenomenon, one of which seeks its origin exclusively in variations in the intensity of terrestrial magnetism, the other in the electricity of the air, the former must give place to the latter, since there are very many convincing proofs in its favor. Unhappily, our knowledge of the electric state of the atmosphere in high polar regions is very limited; could it be extended, all doubts which now exist in regard to the subject would probably disappear. The attempts made to discover the nature of atmospheric electricity in the regions of the extreme north, have in general given only negative results, with the exception, however, of the researches made in the neighborhood of Bossekop by French savants, who, by sending up a kite or an arrow, attached by a conducting wire to an electroscope, to a vertical height of from 30 to 40 yards, have proved the constant presence of positive electricity; but, these observations are too few in number, and were made in a latitude not sufficiently high to be conclusive.

I. One of the most important of my objects in the physical researches of the expedition of 1868 was the study of the phenomena relating to the electricity of the atmosphere; but notwithstanding all my care, I obtained only negative results. As I am convinced that in every case I could account for my want of success, I will briefly describe the experiments I attempted, in order to pass to the observations I had occasion to make in regard to the aurora borealis.

The first experiment, made on the 26th and 28th of August, 1868, on a narrow tongue of land at Kobbe Bay, by means of the electrometer, gave no result, although the observations were made several times a day, and even at night, at the same time that I observed the magnetic instruments. Expecting to find the cause of these negative results either in the insensibility of the instrument or in the nature of the locality, which, closed on three sides by mountains, was open only on the side toward the sea, I determined to modify my instrument, and to look for a more open place for my observations. It was not until the 28th

of September, while the Sophie was anchored at Southgatt, that I could carry this project into execution.

Having made my electrometer more sensitive, I went, on the day above mentioned, between 11 o'clock and noon, to an island situated at the mouth of the Southgatt, and established my instrument on the highest point of the island, 600 feet above the level of the sea. Notwithstanding these precautions I still obtained no certain result. This was possibly on account of the violence of the wind, which produced oscillations in the electrometer, but other observations made on the 7th of October at King's Bay were equally unsuccessful.

Although these experiments were too few in number, and too incomplete to draw from them any positive conclusion whatever, I am convinced that this absence of electric manifestation was due to the peculiar constitution of the air in these regions. A glance over the hygrometric observations shows that the air was almost constantly saturated with moisture, and this moisture did not exist merely in the form of insensible vapor, but also as fog. This circumstance rendered it almost impossible to isolate the instrument, and consequently to obtain the effects of the electricity of tension. We may at least conclude that there is no elec tricity of tension in northern aerial regions which approximate to a plain, but that the electricity rises through damp air into the higher regions of the atmosphere. I am inclined to believe that observations of the electricity of the air made on level ground will always give negative results. Elevated ground should be chosen, and an instrument which may be sent up into the higher strata of the atmosphere, such as the kite used by Franklin.

Setting aside these incomplete experiments, which can only be of use as guides for future efforts, I pass to the observations upon the aurora borealis.

During the last days of September the Sophie was anchored at Southgate, a strait lying between the island called Danes Island and the continent of Spitzbergen, at 79° 39′ 7′′ of latitude and 11° 7′ of longitude west of Greenwich. The gulf of which this strait is an outlet is surrounded on the north and south by mountains, those on the south about 300 meters in height. At the mouth of this gulf lies the island above mentioned; to the east the view is limited by other mountains varying in height. The Sophie was anchored close to the shore of Danskow, a little to the northwest of the island, at the mouth of the strait. On returning from the island, where the instruments for the magnetic observations had been deposited, I perceived upon the ridge of the mountain, to the south, a brilliant polar light rising from 100 to 15° above the mountain in undulating rays, distinctly defined, at their base appearing as a diffuse yellowish light, but higher up as vertical orange beams, while at the top they formed a series of sharp points. The rays had an undulating motion, and the crest of the mountain was covered with a light fog, which the wind was moving from east-northeast to

west-southwest. In a few moments the cloud of mist passed the mount. ain and the rays disappeared, but the crest of the mountain continued to be illuminated by a pale wandering light, which floated along the mountain, and of which it was not easy to determine the character; still I was in no doubt, for the spectral analysis very clearly displayed the yellow ray discovered in polar light by M. Angström. I continued to observe the crest of the mountain, over which foggy vapors were passing, allowing to appear from time to time the pale light I have described. At 11 o'clock 30 minutes the upper part of the fog, which presented very much the appearance of a cloud with serrated edges, became illuminated with a yellowish white light, in the course of a moment converted into yellowish and reddish rays, which extended with an undulatory motion along the edges of the fog, following the irregularities in their minutest detail. The fog rose in the form of an arch about 100 above the mountain, and the rays attained a height of from 100 to 15°, which gives for the whole phenomenon an elevation of from 200 to 250 above the borizon. At the same time there began to appear at the north an indistinct combination of the brilliant edges of clouds, among which I clearly distinguished one, from which proceeded a distinctlymarked yellow ray, seeming in appearance to connect this cloud with another. The rest of the sky was covered with fillets or bands of light clouds, passing over the zenith from the east to the west and allowing the stars to appear at intervals.

The day following, the 26th of September, having observed the crest of the mountain with attention, I found it was almost entirely covered with snow, except at one or two places, which seemed to be those at which the night before the light had appeared with least intensity. The evening of the same day the phenomenon was again manifested, but with some modifications. A little below the horizon to the southwest, almost opposite the promontory or headland which terminates the mountain, appeared a series of clouds whose upper edges were strongly illuminated with a diffuse yellow and white light, which was very intense on the edge of the cloud at the extreme western end of the series, but diminished in brilliancy along the edges of the clouds until at the eastern end, where the last cloud seemed to melt into the headland, it was hardly perceptible. Very soon rays appeared, similar to those observed the night before, which seemed to proceed from a mist lying along the crest of the mountain, but somewhat back of it. This time the phenomenon seemed to take place at a much greater distance than during the preceding observations, but the form and color of the rays were the same; and I again, with the spectroscope, obtained the yellow ray as well from the light emanating from the edge of the clouds as from that proceeding from the rays themselves.

On the 27th of September, after having observed in the morning a radiation of yellowish-white light proceeding from one edge of a cloud which stood out prominently from a wall of clouds, I perceived in the

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