ARTICLE VII. Astronomical Observations, 1823. Bushey Heath, near Stanmore. Latitude 51° 37′ 44.3′′ North. Longitude West in time 1' 20.93′′. SIR, ARTICLE VIII. Essays on the Construction of Sea Harbours. (Concluded from p. 15.) July 18, 1823. WITH regard to the right direction of the piers, so as to throw the most of the surf to the shore, and as little as possible towards the mouth of a harbour; it is necessary first to ascertain the direction of all gales, that raise heavy surves at that place. Land gales at a harbour are harmless in this respect; and so in general are all gales making less angles with the main shore than 15°; so that all dangerous gales as to surf, lie in about 150° directly in front of the harbour. But some situations have not more than 130° of strong surf, and the most leeward of such surf, at the mouth of a harbour facing the calmest quarter, makes an angle of 45° with the main shore. Certain considerations have almost universally prevailed in forming harbours. A sheltered situation has been selected, either in a creek, or near a part of the main shore that projects into the sea. The mouth faces that side which is the least disturbed by a b Fig. 4. sea gales. The same plan, or at least the same principle, has been adopted as to the piers; if in a creek, the main pier, a, fig. 4, begins at the abrupt side, and extends either in a line parallel to the main shore, or inclines a little inwards. This pier has the best direction for quieting the interior water, and preventing the surf from disturbing the entrance, both being taken equally into consideration. But one pier alone placed in any direction can neither sufficiently still the interior water, nor prevent the surf from accumulating at the entrance. For the completion of the former purpose, a secondary pier, b, is built, nearly at right angles to the main pier, extending to the shore from such a distance within the head of the main pier, and leaving an opening for the entrance of such a length, that the line hab of the most leeward heavy surf, points to the outside of the secondary pier's head. These are the inclosing piers, and others for mooring vessels, &c. may be built in the basin thus rendered smooth. To relieve the main pier from as much of the surf as possible, when the abrupt side extends further into the sea, a small covering pier, e, juts out from the extreme point of the land so far that a line, e d, drawn from its sea-end to the head of the main pier, makes an angle of 45° with the direction of the last pier. More than this, the main pier cannot be covered, without interfering too much with the lines of approach; but it keeps back the surves striking under angles of 15° to 45°, which, when strong, are the most dangerous surves. Every surf acting against the secondary pier rebounds to the shore, so that a covering pier is not wanted to protect the entrance from its reverberated water. But heavy surves on a long range of lee-shore force a strong lateral agitation into the harbour. This can be considerably weakened by a very small pier, c, placed at such a distance from the secondary pier, that its sea-end shall not reach the line e d, when continued to the shore. This pier also lessens the quantity of surf in the space between it and the secondary pier, b, and so secures a safer retreat than the exposed shore, to vessels failing to enter the harbour. But a greater length of smooth shore is very desirable, as in particular gales vessels are sometimes driven past the pier, c. Hence also harbours gradually extended have obtained additional works, which make the inclosed lee-shore and entrance more or less similar to those represented by fig. 4, when the dotted pier, g, is added. In this state, vessels can take shelter within either head; but if driven too far in, they can reach the moorings on the innerside of the piers, or retire to the shore, which is much smoother than the exterior shore. In situations where the main pier begins at the extreme point of the abrupt side of a creek, or extends further into the sea; or where it commences from a straight part of the main shore, stretches directly into the sea, and then turns to be parallel to the shore ; a covering pier would be too expensive; and to prevent the reflected surf from accumulating too much at the sea end, the main pier, as in fig. 5, is built in parts not exceeding 100 yards each, having angles of 25° to 30°, with Fig. 5. one another; and turning round bow-wise, sometimes with a head under a less angle. Much of the surf rebounding from one part, does not pass along the others; and what reaches the head is considerably less than would pass to it, if the pier was built in a straight line. Fig. 6. But one long main pier to form a harbour of the first kind, is not so complete as two main piers having an entrance between them, as in fig. 6. This certainly may be considered as two harbours facing each other, and rejecting the surf, but mitting it to pass into the space between them. It has a perfect entrance, as is shown in the first essay; but the other has not, having an open side to the lee-shore, and its entrance is less disturbed by the surf than the head of the long main pier. per ARTICLE IX. Experiments for ascertaining the Velocity of Sound, at Madras in the East Indies. By John Goldingham, Esq. FRS.* BETWEEN the years 1793 and 1796 a considerable number of observations were taken by myself, and under my superintendence, at the Observatory at Madras, with the view of ascertaining the velocity of sound. Not having the exact distances of the guns from the station when I returned to England, I wrote for further information upon the subject-which I had not obtained when I quitted Europe again. I therefore did not bring these experiments forward at the time; and having a more elevated station to observe from, by the erection of a new building, and the advantage of corroborating distances, by the trigonometrical survey carrying on under the superintendence of Col. Lambton, I entered upon the course of experiments about to be detailed. The former experiments (those of 1793 and 1796) were made with Arnold's chronometers, as were these now given. In examining works obtained from libraries here, since I closed these experiments, for information relative to the results of like experiments by other observers, I found a letter from Col. Beaufoy, in the Annals of Philosophy, addressed a few years ago to Dr. Thomson; and recommending to be done in England, what, in all the essential points, has been performed here, as will appear by the following extract: "It has frequently excited my surprise, as well as regret, and in which I am no wise singular, that use has not been made of * Abstracted from the Phil. Trans. for 1823, Part I. the admirable Trigonometrical Survey, begun by the late Gen. Roy, and continued with so much ability and attention by Col. Mudge and Prof. Dalby, to make experiments on the velocity of sound; and however experiments of this kind may have been neglected, it is hoped that the present Master General of the Ordnance, a near relation of the late scientific Capt. Phipps (afterwards Lord Mulgrave) will, for the purpose of perfecting a branch of science, no less curious than useful, order a series of experiments of this nature to be undertaken, not only in the inland parts of the kingdom, but also on different parts of the Coast." He then mentions that the experiments should be made under different circumstances of the wind and weather, and at different times of the 24 hours, and proceeds to enumerate the stations where the experiments should be made. He recommends that pocket chronometers should be used, 'which, generally, making five beats in two seconds, the velocity of sound could be determined to the fraction of a second;' and concludes by saying, he has no doubt scientific foreigners would assist our countrymen in finding the time sound is travelling across that part of the Channel, where the shores are visible from each other."* At Fort St. George (Madras) a morning and an evening gun are fired from the ramparts, as is customary in fortified places, the former at day light, and the latter at eight o'clock in the evening. At St. Thomas's Mount, the artillery cantonment, morning and evening guns are also fired, one at day light, and the other at sun set. The Madras Observatory, in latitude 13° 4′ 8′′ north, is situated between these; the distance of it from the Fort, about half its distance from the Mount, the Fort being to the NE of the Observatory, and the Mount to the SW. In former years, as I have mentioned before, experiments were made by me for ascertaining the velocity of sound, but were not brought forward. And a new building,† elevated so as to give a commanding view of the country, particularly of the Mount and Fort,§ having been erected, I commenced a new series with the morning and evening guns of both places. The experiments with the Mount gun, it will be seen, comprise an interval, which embraces all the varieties of the wind and weather during the revolution of the sun; the interval with the Fort gun is less, in consequence of the morning and evening guns having been fired from different parts of the ramparts, after the date at which the Fort experiments close. All the experiments were made with chronometers, which had 100 beats in 40 seconds, sometimes by three observers, myself and two of the Observatory Bramin assistants, but generally by two: the observers * Annals, O. S. iv. 233. The station on this building is about 55 feet above the level of the sea, distant in a direct line 4500 yards. The Mount gun is about 120 feet above the level of the sea. having repaired to the station at the top of the Observatory building, a little before the expected time, and each holding his chronometer so that he could distinctly hear the beats, began to count the instant he saw the flash, and continued counting until he heard the report; the number of beats between the flash and report was then immediately put down upon a slip of paper, by each observer, without communication with the others, and the papers delivered to me for their contents to be registered; the height of the thermometer, barometer, and hygrometer, with the direction of the wind and state of the weather, were also observed at the time, and registered; and in this manner the whole of the experiments were made. The situations of the guns with respect to the station from which the observations were taken, was very favourable, being in the direction, one of NE, and the other of the SW monsoons-with the southerly wind and sea breeze (both which prevail at certain seasons of the year), blowing between the two. The guns used were 24 pounders, charged with 8 lbs. of powder, and both pointed, not exactly towards the station, but in a direction not far from it. The distances were ascertained with great care; first, by a survey made for the purpose, a base having been measured, and the angles taken with a grand circular instrument, similar to that used on the trigonometrical surveys. Secondly, by using two or three of Col. Lambton's distances and bearings found by the trigonometrical survey. * The results were thus deduced, and verified in different ways; and I have reason to think that the distances of the guns from the Observatory station are very accurately given. The mean of twelve results made the distance of the Mount gun from the station 29547 feet; and the mean of six results gave the distance of the Fort gun from the station 13932-3 feet. We see, as I before remarked, that the distance of one gun from the station is nearly double that of the other, and this will be found an advantage, in showing whether sound travels equally during its progress. The experiments are given in eleven tables.† Table I. Contains the experiments of each day with the Mount gun, together with the state of the atmosphere and the direction of the wind at the time of observation the titles at the heads of the columns render a particular explanation unnecessary the number of observers is stated in the third column, and the mean of their observations in the ninth. Table II. Contains the mean of observations of each day, when the air was calm. * I have not given the details of the survey, as that would swell the paper to an inconvenient size: the base, however, was measured with great care twice, and generally six observations were taken for finding each angle, each observation differing very little from the other. + These Tables are necessarily omitted in this abstract. |