492 REPORT—1868. 
every nineteen years or thereabouts (the period of revolution of the line of 
nodes of the moon’s orbit). Observations continued for nineteen years will 
give the amount of this variation with considerable accuracy, and from it the 
proportion of the effect due to the moon will be distinguished from that due 
to the sun. It is probable that thus a somewhat accurate evaluation of the 
moon’s mass may be arrived at. 
4, The methods of reduction hitherto adopted*, after the example set by 
Laplace and Lubbock, have consisted chiefly, or altogether, in averaging the 
heights and times of high water and low water in certain selected sets of 
groups. Laplace commenced in this way, as the only one for which observa- 
tions made before his time were available. How strong the tendency is to 
pay attention chiefly or exclusively to the times and heights of high and 
low water, is indicated by the title printed at the top of the sheets used 
by the Admiralty to receive the automatic records of the tide-gauges ; for 
instance, “‘ 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 equi-distant 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 practice 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 instant. But direct ob- 
servation and measurement would probably be more accurate than the records 
of the most perfect tide-gauge likely to be realized. 
5. 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 different methods) to come to a resolution as to what method 
should be recommended when new sets of observations are set on foot in any 
place. Inthe 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 be 
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 telegraph 
* See ‘Directions for’ reducing tidal observations,’ by Staff-Commander Burdwood, 
London, 1865, published by the Admiralty; also Professor Haughton on the “Solar and 
Lunar Diurnal Tides on the Coast of Ireland,” Transactions of the Royal Irish Academy 
April for 1854. 
