338 REPORT— 1872. . 



by the Admiralty to receive the automatic records of the tide-gauges ; for 

 instauce, " Diagram, showing time of high and low water at Ramsgate, 

 traced by the tide-gauge." One of the chief practical objects of tidal investi- 

 gation is, of course, to predict the time and height of high water ; but this 

 object is much more easily and accurately attained by the harmonic reduction 

 of observations not confined to high or low water. The best arrangement of 

 observations is to make them at equidistant intervals of time, and to observe 

 simply the height of the water at the moment of observation irrespectively 

 of the time of high or low water. This kind of observation will even be less 

 laborious and less wasteful of time in jjractice than the system of waiting 

 for high or low water, and estimating by. a troublesome interpolation the time 

 of high water, from observations made from ten minutes to ten minutes for 

 some time preceding it and following it. The most complete system of obser- 

 vation is, of course, that of the self-registering tide-gauge, which gives the 

 height of the water-level above a fixed mark every iustant. But dii'ect ob- 

 servation and measurement would probably be more accurate than the records 

 of the most perfect tide-gauge likely to be realized. 



8. One object proposed for the Committee is to estimate the accuracy, 

 both as to time and as to scale of height, attained by the best self-registering 

 tide-gauges at present in use, and (taking into account also the relative 

 costliness of dift'erent methods) to come to a resolution as to w^hat method 

 should be recommended when new sets of observations are set on foot in anj' 

 place. In the mean time the following method of observation is recommended 

 as being more accurate and probably less expensive than the plan of measure- 

 ment on a stem attached to a float, often hitherto followed where there is no 

 self-registering tide-gauge. A metal tube, which need not be more than 2 or 

 3 inches in diameter, is to be fixed vertically in hydrostatic communication, 

 by its lower end, with the sea. A metal scale graduated to centimetres (or 

 to hundredths of a foot, if preferred) is to be let down by the observer in the 

 middle of the tube until it touches the liquid surface ; and a fixed mark 

 attached to the top of the tube then indicates the reading which is to bo 

 taken. Attached to the measuring-scale must be one or more pistons fitting 

 loosely in the tube and guiding the rod so that it may remain, as nearly as 

 may be, in the centre of the tube. The observer will know when its lower 

 end is precisely at the level of the surface of the liquid, by aid of an electric 

 circuit completed through a single galvanic cell, the coil of a common tele- 

 graph " detector," the metal measuring-scale, the liquid, and the metal tube*. 

 Ey this method it will be easy to test the position of the water-level tridy 

 to the tenth of an inch. It is not probable that tidal observations hitherto 

 made, whether with self-registering tide-gauges or by direct observations, 

 have had this degree of accuracy; and it is quite certain that a proper 

 method of reduction will take advantage of all the accui'acy of the plan now 

 proposed. 



9. An observation made on this plan every three hours, from day to day 

 for a month, would probably suffice to give the data required for nautical 

 purposes for any harbour. It is intended immediately to constiiict an appa- 

 ratus of the kind, and give it a trial for a few weeks at some convenient 

 harbour ; and if the plan prove to be successful and convenient, it will come 

 to be considered whether observations made at every hour of the day and 



[* Instead of the galvanic detector, an hydraulic nietliod may be found preferable in some 

 places. The latter consists in using a stiff tube of half inch diameter or so, instead of the 

 .solid metal measuring-bar, and testing whether its lower end is above or below the level of 

 the water by suction at the upper end.] 



