TIDAL OBSERVATIONS. 493 
“ detector,” the metal measuring-scale, the liquid, and the metal tube*. By 
this method it will be easy to test the position of the water-level truly to the 
tenth of aninch. 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 accuracy of the plan now proposedt. 
6. 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 construct 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 
night might not, all things considered (accuracy, economy, and sufficiency for 
all scientific wants), be preferable to a self-registering tide-gauge. 
7. One of the most interesting of the questions that can be proposed in 
reference to the tides is, how much is the earth’s angular velocity diminished 
by them from century to century? although the direct determination of this 
amount, or even a rough estimate of it, can scarcely be hoped for from tidal 
observation, as the data for the quadrature required could not be had directly. 
But accurate observation of amounts and times of the tide on the shores of 
continents and islands of all seas might, with the assistance of improved 
dynamical theory, be fully expected to supply the requisite data for at least 
a rough estimate. In the mean time it may be remarked that one very 
important point of the theory, discovered by Airyt, affords a ready means 
of disentangling some of the complicacy presented by the distribution of the 
times of high water in different places, and will form a sure foundation for 
the practical estimate of a definite part of the whole amount of retardation, 
when the times of spring tides and neap tides are better known for all parts 
of the sea than they are at present. To understand this, imagine a tidal 
spheroid to be constructed by drawing an infinite number of lines perpendi- 
cular to the actual mean sea-level continued under the solid parts of the earth 
which lie above the sea, and equal to the spherical harmonic term or Laplace’s 
function, of the second order, in the development of a discontinuous function 
equal to the height of the sea at any point above the mean level where there 
is sea, and equal to zero for all parts of the earth’s surface oceupied by dry 
land. This spheroid we shall call for brevity the mean tidal spheroid (lunar 
or solar as the case may be, or luni-solar when the heights due to moon and 
sun are added). The fact that the lunar semidiurnal tide is, over nearly the 
whole surface of the sea, greater than the solar, in a greater ratio than that of 
the generating force, renders it almost certain that the longest axes of the mean 
luni-tidal and soli-tidal spheroids would each of them lie in the meridian 90° 
from the disturbing body (moon or sun) if the motion of the water were un- 
opposed by friction; or, which means the same thing, that there would be on 
the average of the whole seas, Jow water when the disturbing body crosses 
the meridian, were the hypothesis of no friction fulfilled. But, as Airy has 
shown, the tendency of friction is to advance the times of low and high water 
[* Instead of the galvanic detector, a hydraulic method 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. | 
[+ The “Clyde Trust” have given permission to try this method at a convenient place 
in the harbour of Glasgow. It is probable that it will also be tried in the harbour of 
Belfast. ] 
t See Airy’s ‘Tides and Waves,’ § 459, 
