« السابقةمتابعة »
secular changes in the relations of land and sea, and that no conclusive results could be obtained unless the observations were carried over a period of rather more than a year at the commencement, and a corresponding period at the close, of the investigation.
He further saw that if this were done, the value of the operations would be greatly increased, because the results would not only serve the purpose for which they were originally contemplated, but would materially contribute towards the attainment of the better knowledge of the law of the tides, which is considered by the British Association to be so important a desideratum, and which is expected to lead to an evaluation of the mass of the moon, to definite information regarding the rigidity of the earth, to an approximation of the depth of the sea from the observed velocities of tide-waves, to the determination of the retardation of the earth’s rotation due to tidal friction, and also to the various practical benefits which necessarily accrue from accurate predictions of the height of the tide at any given time.
Preliminary Preparations.-With the sanction of the Secretary of State for India, Lieut. (now Captain) A. W. Baird, R. E., Assistant Superintendent G. T. Survey, who was then in England on furlough, was deputed to study the practical details of the mode of tidal registration and of the harmonic analysis of the observations, which were recommended and practised by the Tidal Committee of the British Association.
Lieut. Baird also tested at Chatham a new self-registering tide-gauge constructed by Adie, the well-known optician and mathematical instrumentmaker, on the same principle as those he had previously sent out to India, which were provided with barrels of unusual length (five feet) in order that the tidal curves might be drawn on the largest scales practicable. The new tide-gauge was on the same pattern, but with a few modifications, the most important of which was the substitution of a chronometer escapement instead of a pendulum or gravity escapement for the driving clock, in order to permit of the instrument being erected on positions where the concussions of the sea waves would interfere with and perhaps stop the action of a pendulum clock. On trial it was found to work very satisfactorily.
No tidal registrations can be deemed complete without simultaneous registrations of the condition of the atmosphere, because it is well known that the rise and fall of the tides on a line of coast is materially influenced by the direction and force of the winds, and that it also varies inversely with changes in the barometric pressure. Arrangements were therefore made for supplying each tidal station with an anemometer and a barometer, both self-recording The anemometers registered both direction and velocity and were similar to Beckley's, but smaller, in order to be light and portable. The barometers were aneroids, because safely portable selfregistering mercurial barometers could not be obtained.
Selection of Stations.--On Col. Walker's return to India, in November 1872, he deputed Lieut. Baird to make a reconnaisance of the Gulf of Cutch, with a view to selecting tidal stations, and more particularly to ascertain how the instructions which had been received from the Government to establish a station “at a point as far into the Runn of Cutch as possible to which the tide has free access” could be best carried out. For a point to have free access with the sea it is necessary that it should always have at least 4 or 5 feet of water over it at lowest tides, and also that the sea should approach it directly, and not through tortuous channels; the point must also be either on the edge of the mainland or at no great distance beyond, because of the difficulty and expense of constructing a station on the foreshore. It seemed not improbable that it might not be possible to find a point at the edge of the Runn which would satisfy all the requisite conditions.
Lieut. Baird proceeded first to Júria Bandar, close to the head of the Gulf, where he fitted up a country boat for navigating the creeks and channels of the Gulf, and secured the services of an experienced pilot to accompany him in his explorations. After a month's cruising about and long searching along the muddy foreshores of the Gulf, three places well adapted for tidal observations were found. 1st, Nawanár Point, midway up the Gulf on the Cutch coast, 15 miles from Mundra ; 2nd, Hanstal Point at the head of the Gulf, about 18 miles from Júria, and 3rd, Okha Point on the Kattywar coast, just at the mouth of the Gulf, opposite the Island of Beyt. At Nawanár there was a minimum depth of 19 feet of water within 336 feet of a site for a station ; at Hanstal 72 feet within 160 feet of a site for a station, and at Okha 23 feet within 220 feet of a site for a station. Nawanár is about 9 or 10 and Hanstal 16 miles from the nearest village where drinking water can be procured ; Okba Point has Beyt within 1 mile, but a boat is required for communication with it.
The three places selected were considered to be well adapted for the operations, which was the more fortunate in that Lieut. Baird believed them to be the only suitable points to be met with for the purpose. It is to be regretted, however, that an intermediate point could not be found on the Kattywar coast, between Okha and Hanstal, for Nawanár being on the opposite coast had to be connected with the other two stations by a very long line of levels passing round the head of the Gulf; and, as it afterwards turned out, Nawanár proved' unsuitable by constant changes in the configuration of the foreshore.
Preliminary Arrangements. The stations having been selected, preliminary arrangements were commenced.
The first question to be decided was whether the tide-gauges should be set up on stages erected in the sea beyond the low-water line, or
másonry platforms constructed on shore at the high-water line. It is obviously desirable that the communication between the surface of the ocean and the gauge should be as direct as possible, in order that the tidal curve may be accurately delineated. Thus, it is usual to erect tide-gauges in ports or harbours where the piers, quays and landing-stages constructed for the requirements of the shipping present facilities for their being set up in the vicinity of deep water. In the Gulf of Cutch, however, the stations were all at a distance from the nearest inhabited localities and presented no facilities whatever ; for not only building materials and food for the workmen, but even fresh water, had to be brought to them from considerable distances. It was thus imperative that the plan of operations should be of the simplest nature possible, so as to be carried out with the least cost and the greatest expedition. Had any jetties or piers been available for the operations the stations would have been erected on them, but under existing circumstances it was only possible to connect the tidegauges with deep water by erecting stagings for them in the sea ; and these would have had to be very strongly built to withstand the full force of the sea, without undergoing any displacement whatever, and that, not for a short time only, but for several years, so as to include both the first series of tidal registrations, taken to determine the present relations of the land and sea, and the final series which will have to be taken to determine the future relations some years hence. The stagings would, moreover, have had to be connected with the land by piers, in order to permit of ready access to the instruments at all times. The cost of such stagings and their connecting piers would have far exceeded the funds available, and therefore Col. Walker decided, though with some reluctance, on having the tidegauges set up on shore, over wells sunk near the high-water line and connected with the sea by piping.
Final Arrangements.-The following is a brief sketch of the arrangements adopted :
Masonry wells of a diameter of about 3 feet were sunk at the stations to a depth of several feet below the lowest tides ; in these wells iron cylinders with an internal diameter of 22 inches, slightly exceeding the diameter of the float of the tide-gauge, were set up vertically and connected with the sea by an iron piping carried along the shore down to the low-water line, where a flexible piping was attached and carried out into deep water. The flexible piping terminated in a rose suspended by means of buoys a few feet above the bed of the sea, in order to prevent the entrance of silt as much as possible, and was attached to the iron piping in such a manner that it might be readily removed and cleaned whenever necessary. The tide-gauges were set up over the cylinders, and their iron bed-plates indicated the planes to which the tidal measurements were referred; they were connected by
levelling with permanent bench-marks fixed in the ground in the vicinity of the stations.
The iron cylinders were made up in sections of 50 inches in length, a sheet of wrought-iron being bent to the size required and rivetted to form a cylinder, a cast-iron flange was fitted on to each end and the faces of these carefully turned so as to fit exactly. The bottom section had a flat ironi plate carefully screwed on to one end, so as to form the bottom of the well, and the whole when bolted together formed a water-tight well into which water could only enter through the piping connecting it with the sea. The size of the cylinders was decided on so as to utilise the iron sheets most economically, and when finished four men could carry one section. Before being let into the wells they were well painted over, inside and outside, with tar in order to keep them from rusting. The level of the top flange of the cylinder was about 6 feet above high-water spring tide, and about 2 feet 6 inches above the floor of the observatory. A board fitted on and screwed to the top of the cylinder, with holes for the float-bands to pass through, prevented anything falling accidentally inside the cylinder.
The rigid iron piping was ordinary gas-pipe in lengths of about 14 feet, with an internal diameter of 2 inches, which had been computed to be sufficient to permit of the transmission of the tidal wave from the sea to the cylinder in the well without sensible retardation, so that the height of the water in the cylinder should always be the same as that of the surface of the sea. In order to render the connections perfectly water-tight, as well as to facilitate the joining together of the lengths of piping, these were fitted with cast-iron flanges made to screw on to each end.
The piping was connected with the bottom of the cylinder, at 9 inches above it, by a small bend, and was then brought up vertically outside the cylinder to a height 1 or 2 feet below the lowest high-water. At this point was another bend with a stop-cock in it, and the pipe was then taken straight out down to the sea along the slope of the shore to reach low-water springs.
The rigid and flexible pipes were connected together by means of the following arrangement:
To the end of the rigid iron pipe a brass connecting piece, made as shown in the figure, was fitted, having two outer extremities, to one of which a flexible two-inch suction-pipe was fixed and the other closed by a brass disc with a good washer. When the flexible pipe had to be examined for cleaning, the brass disc was unscrewed and a short length of spare flexible piping with a rose at its end fitted on, and taken out to deep water temporarily. The original pipe was then taken off and cleaned, the disc being screwed on for the time in its place ; then when finished the long pipe and disc were replaced in their original positions. The flexible pipe was
two inches in diameter with copper inside, just the same as the suction-pipe. It was provided in lengths of 50 or 60 feet, and in smaller one of 20 feet for temporary use only. The lengths were fitted with couplings and unions for connecting them with each other.
Lengths of this suction-pipe were joined on to the end of the iron piping in the
manner above described, and taken out to disc deep water. At the end of the outer length
a copper rose, of about 15 inches in length, 2 inches in diameter, and having about 150 holes of 3 of an inch bored in it, was screwed on. This rose was sustained a few feet from the bot
tom, being attached to a small nun-buoy by a chain and shackle with swivel, the whole being held in position in deep water by an anchor. To the top of the small nun-buoy a chain was attached, to which was fixed the large buoy floating on the surface, with
plenty of slack chain to allow for lower: nun-buoy rise and fall of tide, and this Rose
buoy also served to mark the position of the flexible pipe. The arrangement will be understood by reference to the annexed
figure. The level of the water in the well should obviously always coincide with that of the sea, otherwise the registrations of the gauge are worthless; it is therefore imperatively necessary to compare the inside and outside levels from time to time, in order to remove all doubt as to the efficiency of the communication between the well and the sea. For this purpose an ordinary gauge was attached to a pile driven into the bed of the sea, and its zero was connected with that of the self-registering gauge over the well, by spirit levelling, and thus a comparison of the levels could be readily made whenever desirable. On taking these comparisons during certain trial observations at Bombay, Captain Baird was surprised to find that while the levels were generally identical, there were occasional large differences which at first could not be accounted for ; eventually, however, he succeeded in tracing them to the accidental presence of air inside the piping: