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Natural Philosophy.

DIFFERENCE OF LONGITUDE BETWEEN GREENWICH AND NEW YORK.

MR. DENT has communicated to the Athenæum the second instance of the successful transport of Chronometers from London to New York, for the purpose of determining the Longitude of these two cities. The first took place in July and August last, and when compared with the result given by M. Daussy, in the Connaissance des Tems, the difference was 2.63 sec. This observation was made in the first voyage of the British Queen across the Atlantic. On her second trip, Mr. Dent sent a second set of four chronometers from London to New York, with the following result:

By this second experiment the difference of longitude
between the Observatory at Greenwich and the City
Hall, New York, is

According to M. Daussy it is

Difference of the two observations

h. m. sec.

4 56 0.24 W.. 4 56 0.72

0 0.48

The difference of the two observations does not, therefore, amount to half a second! This fact is the more gratifying when we consider that these estimates were made independently of each other, "by different observers in different years, and in vessels propelled by different agents."

During the first voyage, all the chronometers showed a difference between the mean travelling rate and the mean stationary rate, which had the remarkable character of being always on the same side, viz., the losing rates were always increased, and the gaining rates always diminished. The same curious fact again occurred in the second voyage. From this circumstance, the longitude of New York was given by each chronometer scarcely enough to the westward in the outward bound voyage, and too much in the homeward one.

The great rapidity and accuracy with which this important branch of nautical inquiry may be pursued over the whole surface of the globe, as the agency of steam shall be extended, is now considered demonstrated. The instances under consideration show that observations may be made connecting very distant countries, and their several results compared in a few weeks-a circumstance of great consequence-for with the diminution of duration in a voyage, proceeds, in a higher degree, the diminution of all the chances and causes of error in chronometrical experiments at sea. Within the space of ninety-nine days, we have seen the British Queen carry chronometers four times across the Atlantic, and give ample time during each of her visits to New York for the necessary observations of rates, &c. objections founded on the idea that the motion of a steam-vessel would

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affect injuriously the more delicate movement of a chronometer are also groundless.

FRODSHAM'S COMPENSATING PENDULum.

Ir is an ordinary pendulum, with a steel rod: over this Mr. Frodsham slips a zinc tube, which passes through a brass bob, and rests on the adjusting screw at the lower end of the rod, the bob being fastened at the centre by two connecting rods of steel to the tube, at the point at which the expansion of the tube is the same as that of the rod; so that, as the steel rod expands downwards, and is lengthened by heat, the zinc tube expands upwards in the same degree: and, therefore, if the lengths of the rod and the tube be rightly proportioned, the pendulum may be regarded as of invariable length. But, as it is seldom found that different specimens of the same metal have precisely the same expansibility, Mr. Frodsham proposes to have several small pieces, or rings of different lengths, cut from the same tube, as correcting pieces, which are to be slipped on or withdrawn, until the length of tube is found that will compensate the pendulum for change of temperature. Mr. Frodsham states, that the hole in the bob through which the zinc tube passes is larger than the tube, but there are brass fillets at both ends, with a hole in each exactly fitting the tube: these fillets are perforated with several small holes to admit the air, so that any change of temperature may not be prevented from affecting the part of the tube which is within the bob. The zinc tube is larger than the steel-rod of the pendulum; fillets being also placed at each end of the tube, with a hole in each, just large enough to let the rod pass through. The tube is pierced with small holes throughout its whole length, to allow the air access to the rod. In the suspending part of the pendulum, Mr. Frodsham directs attention to what he calls an isochronal piece: it is a brass tube about five inches in length, with a slit about an inch in length at the bottom, to form a spring, so as to slide rather stiffly on the rod. At the upper end of the tube is a clasp, which, by means of two screws, is made to embrace the suspending spring; so that after the pendulum has been adjusted to the length for time, the acting part of the suspending spring may be varied.- Proceedings of the British Association; Athenæum.

ON WAVES.

THE Committee on Waves, (Sir John Robison and Mr. Russell,) have reported to the British Association their researches during the past year; which have confirmed or corrected the results formerly obtained by them, and extended their acquaintance with several interesting phenomena. Concerning the first object of their attention-the nature and laws of certain kinds of waves, Mr. Russell, in a previous report, then stated that all the phenomena of the great wave of translation had been obtained, and that he himself had found no difficulty in obtaining the laws of this wave from the equations of M. Laplace, on the hypotheses, that the motion of the water particles was not infinitely small, and the oscillations infinitely small, as had been hitherto supposed, but that they had the magnitude and nature actually found in these experiments. During the past year, considerable progress

has been made in this examination, and confirmations of the truth of these views had been obtained by the labours of Prof. Kolland; who has also introduced the hypothesis of the particles of water having the motions observed in the experiments, viz., a motion of permanent translation in a given course; and although his results did not perfectly accord with the experiments, they had much closer approximated to them than previous examinations, and were to be regarded as additions to the theory of the motion of waves.

In the second subject of inquiry-the connexion between the motion of waves, and the resistance of fluids to the motion of floating bodiesthe phenomenon of vessels at high velocities riding the waves, had been exhibited to an extent never before witnessed. The wave, divided in two by the prow of the vessel, had risen on each side to a height far greater than that even of the vessel itself; it expanded on each side of the vessel from stem to stern, in a broad, unbroken sheet of water, bearing along the vessel, as it were, between a pair of extended gossamer wings, giving, at extremely high velocities, a resistance much less than had hitherto been observed.

The third point concerned the form of a solid of least resistance,* which had been inferred by Mr. Russell, from theory, to be that which he called the Wave Form of vessel. This form was, that the lines of anterior displacement should correspond to the outline of the great wave of translation anteriorly, and to the outline of the posterior wave of replacement, towards the stern; and the truth of the hypothesis had been confirmed by the experiments of the preceding year. A very remarkable practical confirmation is the fact of a large steam-vessel, of 660 tons, with an engine of 220 horse power, being built on the wave principle, and turning out the fastest vessel in Great Britain. This vessel, built as a pleasure yacht, combined the qualities of sailing fast, and carrying a large cargo: she was named the Fire King, and belonged to Mr. Ashton Smith, of Wales.

The last point of inquiry had been the nature and laws of the tidal wave, as propagated along our shores, and up the estuaries of our great rivers. But the nature of its propagation along our shores, after it ceased to be affected by the celestial influence, formed a terrestrial mechanism, with which we were still very imperfectly acquainted. On this subject, the Committee had made simultaneous observations, at thirteen different stations along the Frith of Forth. It was found that there were four tides a day in the Forth instead of two-four high waters and four low waters. Mr. Russell exhibited drawings of these tidal waves, and gave, what he conceived the explanation of a phenomenon, which is, he thinks, much more common than hitherto supposed. It is well known that the tidal wave which brings high water from the Atlantic to the south-western shores of Great Britain, becomes divided into two parts, one of which passes upwards through the English Channel, and the other passing round the west and north of Ireland and Great Britain, brings high water to the east coast of Scotland and to the Frith of Forth. Now, it appears not to have been

* See Year-Book of Facts, 1839, p. 84.

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recollected, that the other wave, after coming up through the English Channel, and bringing, along with the former, high water to London, must pass on northwards, and, in doing so, will enter the Frith of Forth considerably earlier than the northern wave passing southwards. This southern wave, smaller, but earlier than the other, appears to enter the Frith, and may be traced at every station. It is followed up, however, very rapidly, by the great northern wave, and the former moving more slowly than the other, according to the law of the great wave of translation, is overtaken by it at the higher parts of the Frith; and being both greatly exaggerated by the form of that channel, produces the two tides of the Frith of Forth. Mr. Russell expressed his opinion, that the tides in the upper part of the Frith of Forth would be found to rise as high above the mean level of the sea as the tides of the Bristol Channel. The observations on this subject were not, however, completed, but would be finished in the course of next (the present) year.— Athenæum; abridged.

THEORY OF WAVES.

IN illustration of the theory of waves, it is recorded in the Literary Gazette, that, on Sept. 1, a fir-tree, about 40 feet in length, and covered with great barnacles, was drifted across Dover harbour, and towed into that port. It is supposed to have been parted from the American shore by a hurricane, and borne on its long voyage across the Atlantic by the tidal action of the wave.

HEIGHT OF WAVES.

M. ALME, from experiments made in the Bay of Algiers, from December, 1838, to July, 1839, during the continuance of heavy north and north-east winds, concludes: 1st, that the motion of the sea produced by the agitation of the waves, may be sensible 40 yards in depth; 2ndly, that the motion of the bottom is oscillatory; and, 3rdly, that the extent of this oscillation varies slowly from the bottom to the surface.

INFLUENCE OF ATMOSPHERIC PRESSURE ON THE TIDES.

MR. WALKER, Assistant-Master-Attendant in H. M. Dockyard, Devonport, (who has long devoted much time to tidal phenomena,) considering that half-tide levels on oceanic shores, such as those of a large part of Cornwall and Devon, give the equilibrium level of the sea, proposes the following method for obtaining it; which, whatever opinion may be ascertained of the general value of half-tide levels, affords very considerable facility in ascertaining that level at any given place. The mode is: when the barometer stands at its mean annual height, and the air is calm and still, set up a tide-pole, (or select a rock), in some sheltered corner on the coast. Mark upon it the high and low water levels, and half-way between these points will be found the mean level of the sea. Under the above condition, a single observation will give the mean level very nearly; but numerous observations are necessary when great accuracy is required.

Mr. Walker has observed, with respect to the influence of the pressure

of the atmosphere upon the tidal waters on the shores of Cornwall and Devon, that a fall of 1 inch of the mercury in the barometer, corresponds with a rise of 16 inches in the level of the sea, more than would otherwise happen at the same time, under the other general conditions; a rise in the barometer of one inch marking a corresponding fall in the sea-level of 16 inches. This he has found to be the usual rate of such alterations in level; but very sudden changes in the pressure of the atmosphere are accompanied by elevations and depressions equal to 20 inches of sea-water for 1 inch of mercury in the barometer. Regarding the whole pressure of the atmosphere over the globe as a constant quality, all local changes in its weight merely transfer a part of the whole pressure from one part to another; whence he concludes that the subjacent water only flows into, or is displaced from, those areas; where, for the time, the atmospheric pressure is either less or greater than its mean state in accordance with the laws which would govern the conditions of two fluids situated in the manner of the atmosphere and sea. We might account for the difference observed by Mr. Walker, in the amount of depression or elevation of sea-level produced by sudden changes in atmospheric pressure, by considering that a sudden impulse given to the particles of water, either by suddenly increased or diminished weight in the atmosphere, would cause a perpendicular rise or fall in the manner of a wave beyond the height or depth strictly due to the mere change of weight itself.*

As regards the influence of the winds on the mean level upon the south coast of Cornwall and Devon, Mr. Walker observes that east and west winds scarcely affect it; but that southerly winds raise the sea above it from 1 to 10 inches, and off-shore winds depress the water beneath it as much, according to their force. From occurrences, which we have not space to quote, it would appear that, around the shores of the above districts, when the winds which traverse it have considerable force, the levels which would obtain in calms are considerably disturbed; consequently, minor effects of the same kind are caused by less powerful winds, according to their velocity. To obtain, therefore, true heights in this district above the sea, which should correspond, above a level in both channels, supposing such a level to exist, calm weather is essential for accuracy.--From a valuable Report on the Geology of Cornwall, Devon, and West Somerset; by H. F. De la Beche.

A circumstance connected with this subject, of considerable practical value, has been noticed by Mr. Walker. He has found that changes in the height of the water's surface resulting from changes in the pressure of the atmosphere, are often noticed on a good tide-gauge, before the barometer gives notice of any change. Perhaps, something may be due, in those cases observed by Mr. Walker, to the friction of the mercury in the barometer tube. The practical value of the observation is, however, not the less, be the cause of the phenomenon what it may; for if tide-gauges at important dockyards show that a sudden change of level has taken place, indicative of suddenly decreased atmospheric weight, before the barometer has given notice of the same change, all that time which elapses between the notices given by the tide-gauge and the barometer is so much gained; and those engaged with shipping know the value of even a few minutes before the burst of an approaching hurricane.

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