Royal Society. 205 
fide of the EquhwEilal'^ Hb the two points, where the moo^ 
is vertical ; and let K ^ be the great circle, wherein the moon 
appears horizontal ^ it is evident, that a fpheroid defcribed 
upon Hy^and K^ will nearly reprefent the figure of the fea, 
and C/, CD, C F, Ci, will be the heights of the fea in the 
places/; D, F, J, in all which it is high- water 5 and feeing that 
m 12 hours time, by the diurnal rotation of the earth, the 
point F is transferred to /, and d to D, the height of the fea 
C F will be that of the high-water, when the moon is prefent, 
and C/thatof the other nigh-water, when the moon is under 
the earthy which in the cafe of this figure is lefs than the for- 
mer CF 3 and in the oppofite parallel D J, the contrary hap- 
pens ; the rifing of the water being always alternately greater 
and lefs in each place, when it is produced by the moon 
declining fenfibly from the Equino6iiah fhat being the greateft 
of the two high-waters in each diurnal revolution of the moon, 
wherein Hie approaches neareft either to the Zenith or Nadir 
of the place 5 whence it is, that the moon in the northern 
figns, in this part of the world, makes the greateft tides when 
above the earth, and in the fouthern -.figns, when under the 
earth 5 the effedt being always the greateft where the moon is 
fartheft from the horizon, either above or below it 5 and this 
alternate increafe and decreafe of the tides has been obferved 
to hold true on the coaft of England y at Srifrol by Capt. 
Sturmy^ and at Plymouth by Mr. Coleprefs. But the motions 
hitherto mentioned, are fomewhat altered by the libration of 
the water, whereby, the' the a6lion of the luminaries Hiould 
ceafe, the flux and reflux of the fea would for fome time 
continue 5 this confervation of the impreflfed motion diminifhes 
the differences that otherwife would be between the two fub- 
fequent tides, and is the reafon why the higheft fp ring tides 
are not precifely on the new and full-moons, nor the neaps on 
the quarters, but generally they are the third tides after them, 
and ibmetimes later. 
All thefe things would regularly happen, if the whole earth 
were covered with fea to a great depth 3 but by reafon of the 
flioalnefs of fome places, and the narrownefs of the ftreights, 
by which the tides are in many cafes propagated, there arifes a 
great diverfity in the effe6i, and not to be accounted for, 
without an exa6l knowledge of all the circumftances of the 
places 3 as of the pofition of the land, and the breadth and 
depth of the channels by which the tide flows; for a very flow 
and imperceptible motion of the whole body of the water, 
where 
