494, REPORT—1868. 
when the depth and shape of the ocean are such as to make the time of low 
water on the hypothesis of no friction be that of the disturbing body’s transit. 
Now, the well-known fact that the spring tides on the Atlantic coast of 
Europe are about a day or a day and a half after full and change (the times 
of greatest force), and that through nearly the whole sea they are probably 
more or less behind these times, which Airy long ago maintained* to be a 
consequence of friction, would prove that the crowns of the luni-tidal spheroid 
are in advance of those of the soli-tidal spheroid; and therefore that those 
of the latter are less advanced by friction than those of the former. It is 
easily conceived that a knowledge of the heights of the tides and of the 
intervals between the spring tides and the times of greatest force, somewhat 
more extensive than we have at present, would afford data for a rough estimate 
of the proper mean amount of the average interval in question, that is, of the 
interval between the times of high water of the mean luni-tidal and mean 
soli-tidal spheroids. The whole moment of the couple retarding the earth’s 
rotation, in virtue of the lunar tide, must be something more than that caleu- 
lated on the hypothesis that the obliquity of the mean luni-tidal spheroid is 
only equal to the hour-angle corresponding to that interval of time. 
8. We know, however, but little at present regarding the actual time 
of the spring tides in different parts of the ocean, and it is not eyen quite 
certain, although, as Airy remarks, it is extremely probable, that in the 
southern seas they take place at an interval after the full and change, 
although it may be ata less interval than on the Atlantic coast of Europe. 
There must be observations on record (such as those of Sir Thomas Maclear 
at the Cape of Good Hope, which Staffi-Commander Burdwood showed me in 
the Hydrographical Office of the Admiralty) valuable for determining this very 
important element, for ports on all seas where any approach to a knowledge 
of the laws of the tides has been made. 
To collect information on this point from all parts of the world will be 
one of the most interesting parts of the work of the Committee. 
9, Another very interesting subject for inquiry is the lunar fortnightly, or 
solar semiannual, tide, the determination of which will form part of the 
complete harmonic reduction of proper observations made for a sufficient 
time. The amounts of these tides must be very sensible in all places remote 
from the zero linet of either northern or southern hemisphere ; unless the 
solid earth yields very sensibly in its figure to the tide generating forcet. 
Thus it has been calculated that if the earth were perfectly rigid, the sum of 
the rise from lowest to highest at Teneriffe, and simultaneous fall from highest 
to lowest at Iceland, in the lunar fortnightly tides, would amount to 45 
inches. The preliminary trials of plans for harmonic reduction referred to 
below, make it almost certain that hourly observations, continued for a 
month at two such stations as these, would determine the amount of the 
fortnightly tide to a fraction of an inch, in ordinarily favourable cireum- 
stances as to barometric disturbance, and so would give immediate data for 
answering, to some degree of accuracy, the question how much does the solid 
earth really yield to the tide generating force? 
10. A year before proposing to Section A of the British Association the 
appointment of a Committee to. promote tidal investigation, I applied through 
my friend Staff-Commander Moriarty, R.N., for a year’s tidal diagrams of 
* See Airy’s ‘ Tides and Waves,’ § 544. 
+ Thomson and Tait’s ‘ Natural Philosophy,’ § 810. ; 
+ “On the Rigidity of the Earth,’ W. Thomson, Trans. R.S., May 1862; or Thomson 
and Tait’s ‘ Natural Philosophy,’ §§ 832-849. 
