July 17, 1879J 



NATURE 



281 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, June 19. — "Preliminary Note on a New 

 Tide-predicter." ByE. Roberts, F.R.A.S. (Nautical Almanac 

 Office.) Communicated by Prof. G. G. Stokes, M.A., Sec. 

 R.S., &c. 



Tlie Indian Survey Department having undertaken the super- 

 intendence of tide-registration around the whole sea-board of 

 India and at the jiort of Aden, and also the reduction of the 

 observations by the method of harmonic analysis, with the view 

 to the prediction of the tides for the whole of the ports, it 

 became a matter of necessity, in order to save the large outlay 

 which the numerical operation of their prediction would have 

 involved, that an instrument should be constructed to delineate 

 the predictions. 



Accordingly, on the recommendation of the Svirveyor-General 

 of India, I was desired to design, and to undertake the con- 

 struction of, an instrument to include a sufficient number of 

 tide-components to predict the Indian Ocean tides with all the 

 accvu-acy necessary for practical purposes. 



The present machine is the outcome of the recommendation. 



The instrument combines the following twenty tide-com- 

 ponents : — 



The mean lunar semiditUTial ; 



The first and second overtides of the mean lunar semidiurnal ; 



Two elliptic lunar semidiurnal ; 



Two evectional lunar semidiurnal ; 



One variational lunar semidiurnal ; 



The mean solar divu'nal ; 



The mean solar semidiurnal ; 



The lunisolar semidiurnal ; 



The lunisolar diurnal ; 



The lunar diurnal ; 



The solar diurnal ; 



One lunisolar elliptic diurnal ; 



One lunar elliptic diurnal ; 



One compound (Helmholtz) lunisolar semidiurnal ; 



One compound (Helmholtz) lunisolar quarter-diurnal; 



The solar annual ; and 



The solar semiannual. 



Strictly speaking, there is no sensible astronomical tide-com- 

 ponent of twenty-four mean-solar hours' period, but for the pur- 

 poses of prediction it is necessary to include such a term, a very 

 regular and sensible result of this period being obtained in the 

 analysis, due probably to wind or temperature. The same 

 remark applies partially to the solar aimual and the solar semi- 

 annual, the theoretical tides of these periods being very small ; 

 the analysed results, however, are of considerable value, being 

 due probably to the effect of rainfall and the regularity of the 

 monsoons. These three components should, therefore, be re- 

 garded more as meteorological than astronomical. 



The stipulation that the scale for heights should be one inch 

 per foot range for Bombay necessitated a recording barrel of 

 some 18 inches at least ; the actual length adopted for the instru- 

 ment, however, is 22 inches. The delineation of .the curves on 

 such a large scale rendered necessary some modification of the 

 system of excentric pulleys, as fitted on the tide-predicter of the 

 British Association. It was at first contemplated to fit parallel 

 slides only to the larger of the tidal components ; the whole of 

 them have, however, been so provided. 



The chief difficulty in the constructionjof the [machine is the 

 finding, within reasonable limits, of proportions which shall re- 

 present with sufficient accuracy the periods of the several com- 

 ponents, in order that the machine may be used for a consider- 

 able period of prediction — say, for twelve months. Very great 

 success has been attained in this respect in the present instru- 

 ment. For instance, the error of the period of the chief com- 

 ponent (the mean lunar semidiurnal) relatively to the mean solar 

 semidumal is inappreciable dxu'ing a whole year's predictions, 

 amounting to about o°'lo only in a period of fifty years. The 

 largest deviation from strict accuracy is o°''>,T, after a run repre- 

 senting twelve months. This is, however, of one of the very 

 small components, and insensible in its results. This part of the 

 flesign may be therefore regarded as practically perfect. 



Each component is provided for setting with a crank, in which 

 a sliding piece is fitted, carrying a steel guiding-pin. The guiding- 

 pin is thrown out by means of a fine-cut screw and micrometer 

 head. An improved parallel slide, carrying a ptilley, is also 

 fitted to each component. The guiding- pin works between two 



parallel adjustable steel jaws at the back of the pulley frame. 

 The pulley frame is fitted with a balance-weight, so that its 

 centre of gravity is in a vertical Ime through the pulley's axis. 

 The whole slide is counterpoised by a cord and weight, passing 

 over pulleys, in order to relieve the guiding-pin of all strain and 

 to prevent wear. The steel bar of the pulley slide moves freely 

 in two guides driUed out nearly their entire length to reduce the 

 touching parts to a minimum. The other side of the pulley 

 slide is kept in position by a projecting fork or guide, travelling 

 with freedom along a narrow flat brass bar. Both the brass bar 

 guide and the steel rod guide are divided to millimetres ; the 

 brass bar for approximate, and the steel rod for the accurate, 

 adjustment of the throw of the crank-pin, for which purpose the 

 upper guide of the steel rod is furnished with a vernier. The 

 milled head of the micrometer-screw is also divided and may be 

 used with the divisions on the brass bar guide. The pulley 

 frame is movable on its steel rod, for the purposes of the perfect 

 adjustment of the pulley about the centre of motion of the axis 

 of the crank. 



The axis of the crank carries behind the main plate a fine- 

 toothed wheel, fitted on a slotted cone, with a milled nut for 

 clamping the wheel on its axis. The toothed wheel is driven by 

 an endless screw, carrying a bevelled wheel, which is itself 

 driven by another bevelled wheel on one of the four main axes 

 of the machine. The endless screws and main axes are fitted 

 with counter pivots. 



At the back of the machine are fitted the setting dials. Each 

 dial is toothed round its outer' edge and movable round its centre 

 by a pinion for adjustment. The axis of the component projects 

 through the setting dial, and carries a steel pointer for setting. 



A fine flexible wire fixed to a large screw-head passes alter- 

 nately under and over the puUeys of the lower and upper series 

 of components, and carries an ink-bottle at its free end. The 

 ink-bottle, fitted with a fine glass point, travels in a geometrical 

 slide, and is suspended to give just sufficient pressure to ensure 

 contact on the paper of the recording barrel. 



The recording barrel is fitted with brass pins at equidistant 

 intervals to form the time indications on the paper by perfora- 

 tion. An index for setting is fitted behind the machine at the 

 top of the recording barrel. 



The paper, which is continuous and supplied from a reel, 

 passes round two grooved rollers at the back of the main barrel, 

 and is held in position whilst the pins enter the paper, and after 

 receiving the curves is wound round the haul-off drum. The 

 haul-off' drum rests on toothed driving-wheels, and by friction 

 turns and slips to accommodate itself at a proper tension to 

 receive the recorded paper. Motion is given to the whole sys- 

 tem of wheelwork through the horizontal centre main shaft from 

 a system of clockwork driving-gear at the bottom of the machine, 

 the whole being driven by a weight calculated at about 4 cvrt., 

 and controllei by a fan. A warning bell sounds when the 

 weight is nearly run down. The length of the barrel round 

 which the cord is wound is sufficient to give 15,000 turns of the 

 main . shaft. This corresponds to about three months' run of 

 curves, and wiU occupy about one hour to run oS. A year's 

 tide-curves for any port will thus occupy about four hours. 



The setting of the machine for the prediction of the tide- 

 curves of any port for which the tide-components are known is 

 as follows : — The dials are first turned so that the epoch or time 

 of maximum is exactly tmder or above the highest or lowest 

 point according as the component is situated on the upper or 

 lower row of components. The cranks are set vertically (the 

 slotted cone of the wheel on the axis having been first released) 

 and the guide-pin thrown out to its proper range to represent the 

 half-amplitude of the component. The proper positions of the 

 hands having been previously determined by calculation for the 

 time of starting, the hands are set and the slotted cones tight- 

 ened up. The recording barrel is then set to the time and the 

 wheelwork set in motion. The complete setting occupies only 

 a few minutes. 



The large dial in the centre is for showing the progress of the 

 record, which can be marked occasionally to facilitate the entry 

 of the dates on the record after its removal from the machine. 

 A few supplementary pins are inserted in the barrel for the 

 better distinction of the hours. Two speeds of travel can be 

 given to the paper, viz., i inch and J inch per hour. A fixed 

 rod near the recording pen carries ruling pens for the tracing of 

 base lines, such as dock sills, river bars, or mean tide-levels, or 

 if desired can rule the paper throughout its entire depth to 

 represent feet, metres, &c. 



