Proceedings of the British Association. 363 
than the mean when the moon passes nine hours after the sun; and 
the quantity by which the lunitidal interval is Jess than the mean 
when the moon is three hours after the sun, is exactly equal to the 
quantity by which the lunitidal interval is greater than the mean 
when the moon passes nine hours after the sun. And this equality 
of the defect and excess of the interval at three hours and at nine 
hours of the moon’s transit, is still true where the moon’s force al- 
ters by the alteration of her parallax or declination. Now we are 
to inquire whether this equality of excess and defect of the interval 
in all changes of declination, &c., is exhibited by observation. It 
appears at first sight, that the-equality does not exist; that is, if we 
obtain the lunitidal interval by comparing the tide with the nearest 
preceding transit. But, in truth, we ought not to refer the tide to 
such a transit, because we know that the tide of our shores must be 
produced in a great measure by the tide which revolves in the 
Southern Ocean, and which every half day sends off tides along the 
Atlantic. The tide, therefore, which reaches Bristol, is the result 
of a tide wave, which was produced by the action of the sun and 
moon at some anterior period. It is found, that if at Bristol we re- 
fer each tide to the transit of the moon, which took place about forty 
four hours previously, we do obtain an accordance of the observa- 
tions with theory in the feature above described,—that although the 
moon’s force alters by the alteration of her declination, the defect of 
the lunitidal interval for a three hours’ transit of the moon is equal 
to the excess of that interval for a nine hours’ transit. And thus, in 
this respect at least, the tide at Bristol agrees exactly with the tide 
which would be produced, if, forty four hours before the tide, the 
waters of the ocean assumed the form of the spheroid of equilibrium 
due to the forces of the moon and sun, and if this tide were trans- 
mitted unaltered to Bristol in those forty four hours. 
Wednesday, Aug. 24. 
Section A.—MatruHemMaTicaL and PuysicaL Science. 
The first paper read was by Mr. W. Snow Harris, ‘On some 
phenomena of electrical repulsion.” 
Prof. Challis read his ‘Supplementary report upon the mathe- 
matical theory of fluids.” 
Prof. Stevelly gave his “Illustration of the meaning of the doubt- 
ful algebraic sign in certain formule of algebraic geometry.” 
