ASTRONOMY. 
moon, acting unequally upon different parts 
of the ocean, must occasion it to assume a 
different figure from what it would other- 
wise have, if there were no such unequal at- 
(ractions. And since this attractive force 
is greatest on the part of the ocean which 
lies immediately under the moon, the wa- 
ters will of course flow constantly to that 
part, and be elevated or depressed at dif- 
ferent places, according as her situation 
changes with respect to those places. But, 
as the earth turns round on its axis, from 
the moon to the moon again, in about 
twenty-four hours and three quarters, the 
flux and reflux will be necessarily retarded 
from day to day about three quarters of an 
hour, which is agreeable to experience. It 
remains now to be explained, why they ebb 
and flow twice a day, or in the space of 
about twenty-four hours. When the moon 
passes the meridian of any place, or is at her 
greatest height above the horizon of that 
place, she will evidently attract and elevate 
the waters which lie immediately under her : 
but what is the reason, that twelve hours af- 
terwards, when she passes the meridian be- 
low the horizon, the waters at the same 
place are then also elevated? We know, 
from experience, that, whether the moon be 
in the zenith or nadir, the phenomenon is 
nearly the same; it being high water with 
us at the same time that it is high water with 
our antipodes. 
Let M, (fig. 4.) represent the moon as be- 
fore; O, the centre of the earth ; and Z and 
N, tliose parts of the surface which are the 
nearest to the moon, and the farthest from 
her. Then because the point Z is nearer 
to the moon than any other part of the he- 
misphere H Z R, it is evident that the wa- 
ters will be more strongly attracted by her, 
about that point, than at others which are 
more remote ; and since this attraction acts 
in a contrary direction to that of the earth, 
the waters in all parts, from HR to Z, 
must have their gravity or tendency towards 
the centre O diminished ; and as this ten- 
dency is the least at the point Z, they will 
consequently stand higher there than in any 
other part of the hemisphere. Again in the 
opposite hemisphere HNR, although the 
attraction of the moon conspires with that 
of the earth, yet as it is known to decrease 
in proportion as the squares of the distances 
increase, it is plain that the joint influence 
of the two force?, taken together, will be 
less at the point N, on the side opposite to 
the moon, than at those parts which lie 
nearer to H R, and consequently, as the 
gravity of the waters, or their tendency to- 
wards the centre, is also the least at that 
point, they will be more elevated there than 
in any other part of the hemisphere ; so that 
the attractive force of the moon will evi- 
dently raise the waters, both at that point 
of the surface which is nearest to her, and 
at that which is farthest from her, at the 
same time, as was to be shewn. 
Following this system, then, it is to be 
observed, that at any port or harbour 
which lies open to the ocean, the action of 
the moon will -tend to elevate the waters 
there, when she is on the meridian of that 
place, whether it be above the horizon or 
below it. But the water cannot be raised 
at one place, without flowing from and be- 
ing depressed at another; and these eleva- 
tions and depressions will obviously be the 
greatest at opposite points of the earth’s 
surface. When the moon raises the waters 
at Z and N, they will be depressed at 
H and R ; and when they are raised by her 
at H and R, they will be depressed at Z 
and N. And as the moon passes over the 
meridian, and is in the horizon twice every 
day, there will therefore be two tides of 
flood, and two of ebb, in that time, at the 
interval of about six hours and eleven mi- 
nutes each ; which is exactly conformable 
to theory and experience. 
From what has been hitherto said, it may 
be supposed, that the moon is the sole agent 
concerned in producing the tides. But it 
will be necessary to observe, before we quit 
the subject, that the influence of the sun 
would also produce a similar effect, though 
in a much less degree, than from his supe- 
rior magnitude we should naturally be led to 
imagine. For it is not the entire actions of 
those bodies upon the whole globe of the 
earth that is here to be considered, but only 
the inequalities of those actions upon dif- 
ferent parts of it. The whole attractive 
force of the sun is far superior to that of the 
moon ; but as his distance from the earth is 
nearly 400 times greater, the forces with 
which he acts upon different parts of it, 
will be much nearer to equality than those 
of the moon ; and consequently will have a 
less effect in producing any change of its fi- 
gure. For it is to be observed, that if all 
parts of the earth were equally attracted, 
they would suffer but little change in their 
mutual situations. That this doctrine may 
be still more clearly understood, let it be. 
considered, that though the earth’s diame- 
ter bears a considerable proportion to the 
distance of the earth from the moon, yet 
this diameter is almost nothing when com- 
pared to the distance of the earth from the 
