Imagine a pendulum with single tooth escapement mounted on a collar loose on the escapement-shaft just as described above, the shaft being vertical in this case also. A square-threaded screw is cut on the upper quarter of the length of the shaft, this being the part of it on which the escapement-collar works; and a pin fixed to the collar projects inwards to the furrow of the screw, so that, if the collar is turned relatively to the shaft, it will be carried along as the nut of a screw, but with less friction than an ordinary nut. Below the screw and long nut-collar three-quarters of the length of the escapement-shaft is surrounded by a tube which by wheel-work is carried round about 5 per cent. faster than the central shaft. This outer shaft, by means of friction produced by the pressure of proper springs, carries the nut-collar round along with it, except when the escapement-tooth is stopped by either of the pallets attached to the pendulum. A stiff cross-piece (like the head of a T), projecting each way from the top of the tubular shaft, carries, hanging down from it, the governing masses of a centrifugal friction governor. These masses are drawn towards the axis by springs, the inner ends of which are acted on by the nut-collar, so that the higher or the lower the latter is in its range the springs pull the masses inwards with less or more force. A fixed metal ring coaxial with the main shaft holds the governing masses in when their centrifugal forces exceed the forces of the springs, and resists the motion by forces of friction increasing approximately in simple proportion to the excess of the speed above that which just balances the forces of the springs. As long as the escapement-tooth is unresisted the nut-collar is carried round with the quicker motion of the outer tubular shaft, and so it screws upwards, diminishing the force of the springs. Once every semiperiod of the pendulum it is held back by either pallet, and the nut-collar screws down as much as it rose during the preceding interval of freedom, when the action is regular; and the central or main escapement-shaft turns in the same period as the tooth, being the period of the pendulum. If through increase or diminution of the driving-power, or diminution or increase of the coefficient of friction between the governing masses and the ring on which they press, the shaft tends to turn faster or slower, the nut-collar works its way down or up the screw, until the governor is again regulated, and gives the same speed in the altered circumstances. It is easy to arrange that a large amount of regulating power shall be implied in a single turn of the nut-collar relatively to the central shaft, and yet that the periodic application and removal of about of this amount in the halfperiod of the pendulum shall cause but a very small periodic variation in the speed. The latter important condition is secured by the great moment of inertia of the governing masses themselves round the main shaft. My communication to the Royal Society ended as follows:

"I hope after a few months trial to be able to present a satisfactory report of the performance of the clock now completed according to the principles explained above. As many of the details of execution may become modified after practical trial, it is unnecessary that I should describe them minutely at present. Its general appearance, and the arrangement of its characteristic parts, may be understood from the photograph now laid before the Society."

I am sorry to say that the hope here expressed has not hitherto been realized. Year after year passed producing only more or less of radical reform in various mechanical details of the governor and of the fine movement, until about six months ago, when, for the first time, I had all except the pendulums in approximately satisfactory condition. By that time I had discovered that my choice of zinc and platinum for the temperature compensation and lead for the weight of the pendufums was a mistake. I had fallen into it about ten years ago through being informed that in Russia the gridiron pendulum had been reverted to because of the difficulty of getting equality of temperature throughout the length of the pendulum; and without stopping to perceive that the right way to deal with this difficulty was to face it and take means of securing practical equality of temperature throughout the length of the pendulum (which it is obvious may be done by simple enough appliances), I devised a pendulum in which the compensation is produced by a stiff tube of zinc and a platinum wire placed nearly parallel each to the other throughout the length of the pendulum; and the two pendulums of the clock shown to the British Association were constructed on this plan. Now it is clear

that the materials chosen for compensation should, of all those not otherwise objectionable, be those of greatest and of least expansibility. Therefore, certainly, glass or platinum ought to be one of the materials; and the steel of the ordinary astronomical mercury pendulum is a mistake. Mercury ought to be the other (its cubic expansion being six times the linear expansion of zinc), unless the capillary uncertainty of the mercury surface lead to irregular changes in the rate of the pendulum. The weight of the pendulum ought to be of material of the greatest specific gravity attainable, at all events unless the whole is to be mounted in an airtight case, because one of the chief errors of the best existing pendulums is that depending on the variations of barometric pressure. The expense of platinum puts it out of the question for the weight of the pendulum, even although the use of mercury for the temperature compensation did not also give mercury for the weight. Thus even though as good compensation could be got by zinc and platinum as by any other means, mercury ought, on account of its superior specific gravity, to be preferred to lead for the weight of the pendulum.

I have accordingly now made several pendulums (for tide-gauges) with no other material in the moving part than glass and mercury, with rounded knife-edges of agate for the fixed support; and I am on the point of making four more for two new clocks which I am having made on the plan which forms the subject of this communication. I have had no opportunity hitherto of testing the performance of any of these pendulums; but their action seems very promising of good results, and the only untoward circumstance which has hitherto appeared in connexion with them has been breakages of the glass in two attempts to have one carried safely to Genoa for a tide-gauge made by Mr. White to an order for the Italian Government.

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As to the accuracy of my new clock, it is enough to look at the pendulum vibrating with perfect steadiness, from month to month, through a range of half a centimetre on each side of its middle position, with its pallets only touched during 3 of the time by the escapement-tooth, to feel certain that, if the best ordinary astronomical clock owes any of its irregularities to variations of range of its pendulum, or to impulses and friction of its escapement-wheel, the new clock must, when tried with an equally good pendulum, prove more regular. I hope soon to have it tried with a better pendulum than that of any astronomical clock hitherto made; and if it then shows irregularities amounting to of those of the best astronomical clocks, the next step must be to inclose it in an air-tight case kept at constant temperature, day and night, summer and winter.

On Mr. Sabine's Method of Measuring small Intervals of Time.
By W. H. WALENN,

On Tidal Operations in the Gulf of Cutch by the Great Trigonometrical Survey of India. By Capt. A. W. BAIRD, R.E.

The primary object of the operations was to determine whether secular changes in the level of the land at the head of the Gulf, i. e. the "Runn of Cutch," are taking place. Col. Walker, the Superintendent of the Great Trigonometrical Survey of India, at first intended to restrict the observations to a few weeks duration; but he found that by extending them to a period of a little over a year, scientific results of the highest value would be obtained, and also that this course would be necessary in order to obtain data sufficient to detect minute changes in the relative level of land and sea. I was deputed when in England in 1871 to study the details of tidal observations and harmonic analysis as recommended by the British Association; at the same time I tested a new self-registering tide-gauge, the performances of which were very satisfactory. The self-registering tide-gauges were then described at length, the most remarkable feature in them being the unusually

long barrels (length 5 feet), which were provided in order to give the tidal curves on the diagram on a very large scale. Six of these instruments had been sent out to India some years before; they were modified in Bombay, so as to be similar to the new one which was tested at Chatham, and had scales of wheels put on for adaptation to particular tides, friction-rollers for the barrel, zero-lines for time and height cut, &c. As the rise and fall of the tide is materially influenced by direction and force of the wind, and also by changes in the barometer, self-registering anemometers and barometers were procured for each tidal station. On my return to India I was ordered to make a reconnaissance of the Gulf of Cutch, to select sites for tidal stations. I cruised about for a month in a common native sailing-boat, and after long searching along the muddy foreshores of the Gulf I found three places well adapted for tidal observations-one right at the head of the Gulf, just in the Runn of Cutch, called Haustal Tidal Station; another midway up the Gulf of the Cutch coast, called Nowanar; and the third, Okha, just at the mouth of the Gulf, opposite the island of Beyt. They were well situated for the purposes required, as far as their geographical position; but as one was at a point twenty miles from the nearest village (from which drinking-water had to be brought by boat as well as food for the men in charge), another station nine miles, and the third two miles from the villages, the arrangements for the continuous working of the stations for about a year and a half had to be most carefully made. I returned to Bombay and got all the apparatus ready, such as iron cylinders in length (so as to be portable), iron piping, suction-piping, anchors, and buoys, &c. for the deep-sea connexion, temporary tide-gauges for comparison, portable observatories-in fact every thing, even to bricks and lime for sinking the masonry well for holding the cylinders, for nothing could be procured in the places selected for the stations. While in Bombay I tested the working of the whole apparatus for each observatory, and made many modifications from time to time. found that air would collect in the pipes, which were in the shape of a long siphon, and thus cause differences of level in the cylinder and the sea. I overcame this difficulty by inserting stopcocks at the top bends, which were to be always below the lowest high-water; and in this way I was able to get the same level of the water inside the cylinder as in the open sea. By frequent comparison with the temporary tide-gauges, the identity of level was determined; the size of the pipe connecting the cylinder had been calculated, so that practically there would be no retardation in the flow of the water. The native sub-surveyors, who were to be in charge of the stations, were also trained in Bombay.

The observations and apparatus were then described at length, and several illustrations and diagrams showed the method of their working. In addition to the self-registering anemometer and self-registering aneroid barometer, each observatory was fitted out with a standard mercurial barometer (for comparisons) and a raingauge. Three bench mark-stones in masonry platforms, at different distances from the observatory, were built as standard points for the levels, and each carefully connected with the zero of the self-registering tide-gauge. The whole of the apparatus and instruments were sent off in a large native sailing-vessel direct to Okha, the natives who were to be employed also going. I marched across Kattyanon to Okha, having made some arrangements with the Political Agent at Rajkot as to the help we should get from the native states. The construction of Okha tidal station was then described, and many of the difficulties which were successfully overcome; also the different methods of comparing position of pencil on diagram with the height of water; checks on the working of the instruments for insertion in the daily reports submitted by the sub-surveyors. I detected a serious fault in the selfregistering tide-gauge, viz. that the instrument was by no means correct in the time registration. I eventually devised a simple, plan which I called "back-lash weight," which completely removed this cause of error: I am of opinion this plan ought to be carried out in its entirety, and the barrel made to drive the clock instead of the clock the barrel.

Just after all was ready, and the instruments being started at Okha, a great disaster happened early one morning. A boat drifted down past the station, her anchor dragged across the flexible pipe, smashed it, and carried off a large portion of it, as well as buoys, anchor, &c. Here we had to land and to have the repairs quickly

executed; then the final measurements for determination of zero, rating of clocks, & c. were made, and the instrments started on their eighteen months' work. Leaving Okha, the vessel in which I and my men and all the apparatus were in ran straight on to a sandbank and nearly capsized. After many troubles, the other two stations were eventually constructed. Huts had to be made for the men in charge and the guard from the native state to live in, a regular service for sending food and water established, and post-runners started to carry the daily reports to the nearest post-office, and many other details arranged. I or my European assistant had to make frequent tours of inspection of the stations while work was going on, which entailed much hard marching and exposure. One journey (in May 1874) was described in which I and my assistant had to ride on camels over about fourteen miles of the Runn, covered with water from 6 inches to a foot deep, in order to reach Haustal Tidal Station. The working of the stations was then described, Okha and Haustal giving perfect and continuous registration; but at Nowanar, where there was 20 feet of water at the end of the pipe at low-water in April 1874, in the following July it silted up and buried the pipe, and the whole configuration of the foreshore altered. New pipes had to be got up, and two lunations (from March to May 1875) were secured, in addition to the one and half lunation got before the shore had altered, in 1874. The registrations of the anemograph and barograph were continuous also. The levelling operations (750 miles of double levelling were done in connexion with the work) were next noticed; the rigid method of procedure which obtains in the Great Trigonometrical Survey of India, and which give such wonderfully accurate results, was referred to (vide Col. Walker's paper in vol. xxxiii. of the Memoirs of the Astron. Society).

The reductions of the tidal observations are in progress, some idea of the magnitude of which may be imagined when 30,000 points have been corrected to true mean local time on the diagramn-sheets, corrections made for zero error, and then the 30,000 final measurements made and tabulated for reduction. The determination of the mean level of the sea at each station and some of the results already deduced are stated: one is important, and that is, that the mean level deduced from the two months (March 7 to May 7) is nearly identical with the mean of the whole ear; and this Col. Walker had predicted would be the case in a letter on the subject about eight years before. The meteorological reductions are in progress. The movement of the wind for each hour for the whole period has been tabulated and reduced to its N. and E. components, the mean hourly value determined; and, by combining the differences of this mean from the value of each particular hour, and similarly the barometric differences with the differences of the theoretical and actual values of the tide, I hope to determine far more accurately than has yet been done the effect of the wind and barometer on the tide. Several tracings of the actual diagrams were exhibited. The tidal curves are most regular and continuous, and show the perfect working of the whole apparatus; and when the tidal and meteorological reductions are complete, I hope to obtain some very valuable results.

Physical Explanation of the Mackerel Sky.

By Sir W. THOMSON, D.C.L., F.R.S.

On Navigational Deep-sea Soundings in a Ship moving at High Speed.
By Sir W. THOMSON, D.C.L., F.R.S.

CHEMISTRY.

Address by WILLIAM HENRY PERKIN, F.R.S., President of the Section.

THERE can be no doubt that chemistry and the allied sciences are now being recognized to a much greater extent in this country than in former years; and not only so, the workers at research, though still small in number, are more numerous than they were.

In 1868 Dr. Frankland, in his Address to this Section at the Meeting at Norwich, commented upon the small amount of original research then being carried on in the United Kingdom; but, judging from the statistics of the Chemical Society, this state of things became even worse; for in 1868 there were forty-eight papers read before the Society, but in 1872 only twenty-two. Since then, however, there has been a considerable increase in the number; and at the Anniversary Meeting in March last it was shown that the number of communications for the session had risen to sixty-six, or three times as many as in 1872.

Of course these figures only refer to the Chemical Society; but I think they may be taken as a very safe criterion of the improved state of things, though it would be very gratifying to see much greater activity.

It is also very pleasing to find that the aids to and opportunities for research are increasing, because it must be remembered that, in a pecuniary sense, science is far from being its own rewarder at the time its truths are being studied, although the results very often become eventually of the greatest practical value; hence the wisdom of a country encouraging scientific research.

But little, however, has been done in this direction in past years-the grants made for general science by this Association, and that of the Government of one thousand pounds annually to the Royal Society, being the most important.

The Chemical Society has also been in the habit of giving small grants for the purpose of assisting those engaged in chemical research. In the future, however, it will be able to do much more than hitherto. One of the original members of the Society, Dr. Longstaff, offered in the early part of the year to give one thousand pounds provided a similar sum could be raised, the united amount to be invested and the interest applied for the encouragement of research. I am happy to say that rather more than the required sum has been raised, and it is hoped that it may be still further supplemented.

In addition to the Royal-Society grant, the Government have given this year a further annual sum of four thousand pounds. Of course this is for science generally.

Mr. T. J. Phillips Jodrell has also placed at the disposal of the Royal Society the munificent sum of six thousand pounds to be applied in any manner that they may consider for the time being most conducive to the encouragement of research in Physical sciences.

When we consider how much of our science is of a physical nature, we must be grateful for this bequest; and it is to be hoped that these helps will more and more stimulate research in the United Kingdom; and if we have any hope of keeping pace with the large amount of work now being carried on in other countries, we must indeed be energetic.

The employment of well-trained chemists in chemical works is now becoming much more general than heretofore, especially on the continent, where in some cases a considerable staff is employed and provided with suitable appliances, &c., for the purpose not only of attending to and perfecting the ordinary operations which are in use, but to make investigations in relation to the class of manufacture they are engaged in. A conviction of the necessity of this is gaining strength in this country, though not so quickly as might be desired; nevertheless these things are encouraging.

With reference to the progress of chemistry and what have been the fruits of research of late years, it will be impossible for me to give even a general outline, the amount of work being so large; in fact, to recount the list of investigations made during the past year would take up most of the time at my disposal.