84 
THE REV. W. WHEWELL ON THE 
It appears that in all these cases the mean height of the sea is very nearly constant. 
This is most remarkable at Singapore, where, though the successive low waters often 
differ by six feet, the mean level only oscillates through a few inches. At Plymouth 
the mean level is not quite so steady. The fact is, that at that port the low water 
varies more by the difference of springs and neaps than the high water does ; and 
hence the mean level slightly follows the low water, and is lowest at spring tides, and 
highest at neap tides, or perhaps more exactly a day or two later. 
‘ £ The level of the sea at low water,” a phrase sometimes used by surveyors, is al- 
together erroneous, and may lead to material error. From the instances just quoted 
(and indeed from the nature of the case) it is certain that the mean height of the sea 
is far more nearly constant than low or high water, under whatever assumed condi- 
tions. A level surface drawn from any point (that is a surface of stagnant water) 
would probably be nearly parallel to the points of mean water at different places. 
This becomes more manifest when we consider that at places near each other the tide 
often differs greatly in amount. At St. David’s Head in Pembrokeshire the range of 
the tides is near thirty feet ; on the opposite coast of Ireland it is only two or three: 
if the sea were level at low water the difference of the mean heights on the two sides 
of the Channel (which is only about fifty miles) would be fourteen feet. Such an 
average elevation of one side of a narrow sea above the other is quite inconsistent 
with the laws of fluids. 
I cannot conclude this paper without again pointing out that a great number of 
curious facts in fluid motion are established by these Tide Researches, of which it 
may be hoped the theory of hydrodynamics will one day be able to render a reason. 
Why is it that at places near each other the range of the oscillations of the sea from 
low r to high water is so different ? Why is it that the sun affects the low water at Ply- 
mouth more than the high water, and that the moon’s declination at Singapore affects 
the low water four times as much as the high water, while at Plymouth it affects it 
less ? Above all, why is it that while the effect of the sun, and of the moon’s declina- 
tion and parallax, in the monthly course of the tides, produces the effect due to the 
equilibrium of the forces in one or two days, the moon’s declination does not produce 
its effect upon the diurnal oscillation till after three, four, five, and six days ; and in 
some cases probably not till the moon is exerting forces which tend absolutely to re- 
verse the effect ? 
Table of the Diurnal Inequality of the Height of High Water at Plymouth. 
To be used with the moon’s declination four days anterior. 
For N. decl., add to the tide following moon’s S. transit, subtract from the tide following moon’s N. transit. 
For S. decl., subtract from the tide following moon’s S. transit, add to the tide following moon’s N. transit. 
Moon’s De- "1 
clination / 
0° to 4° 
5° to 9° 
10° to 14° 
15° to 18° 
19° to 21° 
22° to 24° 
25° to 26° 
27° to 28° 
29° 
30° 
Diurnal In- 1 
equality / 
Qin 
l in 
2 >n 
3 in 
^.in 
5 in 
6 in 
yin 
8 in 
gin 
