will passthrough M, and fts shorter a^ist R O F 
will terminate' in B and F, Let the ring be 
ASTRONOMY*. 
filled with fluid particles, so as to form a 
sphere round O; then, as the whole moves 
towards M, the fluid sphere being lengthened 
at Z and n will assume an oblong or oval 
form. It M is the Moon, () the Earth’s cen- 
tre, A B C 1) E F G II the sea covering -the 
earth’s, surface, it is evident, by the above 
reasoning, that whilst the Earth by its gravity 
fulls towards the Moon, the water directly be- 
low at B will swell and rise gradually towards 
her; also the water at D will recede from the 
centre, (strictly speaking, the centre recedes 
from D) and rise on the opposite side of the 
Earth ; whilst the water at B and F is depress- 
ed, and tails below the former level. Hence as 
the Earth turns round its axis from the Moon 
to the Moon. again in 24^ hours, there will he 
two tides ot flood and two of ebb in that time, 
as we line! by experience, 
1 hat (his doctrine may be still more dearly 
understood, let it be considered, that although 
fhe Earth’s diameter bears a considerable 
proportion to the distance of the Earth from 
the Moon, yet this diameter is almost nothing 
; when compared to the distance of the Earth 
from the Sun. lhe difference of the Sun’s 
I attraction, therefore, on the sides of the 
Earth under and opposite to him, will be 
: much less than the difference of the Moon’s 
attraction on the sides of the Earth under and 
opposite to her ; and, for this reason, the 
Moon must raise the tides much higher than 
they can be raised by the Sun. Newton has 
calculated the effect of the Sun’s influence, in 
this case, and found that it is about three 
tunes less than that of the Moon. The action 
ot the Sun alone would, therefore, be sufficient 
to produce a flux and reflux of the sea; but 
the elevations and depressions occasioned by 
this means would be about three times less 
than those produced by the Moon. 
'1 he tides, then, are not the sole produc- 
tion of the Moon, but of the joint forces of the 
S*m and Moon together. Or, properly 
speaking, there are two tides, a solar one, anti 
n lunar one, which have a joint or opposite 
effect, according to the situation of the bodies 
which produce them. When the actions of 
the Sun and Moon conspire together, as at 
the time ot new and full Moon, the flux and 
reilux become more considerable; and these 
are then called the Spring Tides. But one 
tends to elevate the waters, whilst the other 
depresses them, as at the Moon’s first and 
third quarters, and then the effect will be ex- 
actly the contrary ; the flux and reflux, in- 
stead of being augmented, as before, will now 
be diminished ; and these are called the Neap 
fides. 
1 o explain this more completely, let S 
represent the Sun, (tig. 18) Z H R the 
Earth, and 1* and C the Moon at her dull and 
change. Then, because the Sun S, and the 
new Moon C, are nearly in the same right 
hue with the centre of the Earth O, their ac- 
tions will conspire together, and raise the water 
above the zenith Z, or the point immediately 
under them, to a greater height than if only 
one of these forces acted alone. But it. has been 
shewn that when the ocean is elevated to the 
zenith Z, it is also elevated at the opposite 
point, or nadir, at r the same time ; and 
therefore, in this situation of the Sun and 
Moon, the tides will be augmented. And 
again, whilst the full Moon F raises the waters 
VOL. I. 
nt N and Z, directly under and opposite to 
her, the Sun S, acting in the same right line, 
will also raise the waters at the same 'point Z 
and N, directly under and opposite to him ; 
and therefore, in this situation also, the tides 
will he augmented; their joint effect being 
neady the same at the change as at the full"; 
and in both cases they occasion what are called 
the Spring Tides, 
t lld s theory, the tides ought to be highest 
directly under and opposite to the Moon; 
that is, when the Moon is due north and 
south. But we find that in open seas, where 
the waters flow freely, the A loon is generally 
past the north, and south meridian of the 
place where it is high water. The reason is 
ob\ ious ; tor though the Aloon’s attraction 
was to cease altogether when she was past 
the meridian, yet the motion of ascent com- 
municated to the water before that time would 
make it continue to rise for some time after ; 
pinch more must it do so when the attraction 
is only diminished ; as a little impulse given 
to a moving hall will cause it still to move 
farther than otherwise it could have done; 
and as experience shews that tire day is hotter 
about three in the afternoon, than when the 
Sun is on the meridian, because of the in- 
crease made to the heat already imparted. 
Tides answer not always to tire same dis- 
tance of the Moon from the meridian at the 
same places, but are variously affected by the 
action of the Sun, which brings them on 
sooner when the Moon is in her first and third 
quarters, and keeps them back later when she 
is in her third and fourth; because, in the 
former case, the tide raised by the Sun alone 
would be earlier than the tides raised by the 
Aloon; and in the latter case later. 
.The Sun, being nearer our hemisphere in 
winter than in summer, is ot course nearer to 
it in February and October than in March 
and September, and therefore the greatest 
tides happen not till some time after the au- 
tumnal equinox, and return a little before the 
vernal. 
1 17 
. r I he sea, being put in motion, would con- 
tinue to ebb and flow for several times, even 
though the Sun and Aloon were annihilated, 
and their influence should cease ; as if a bason 
of water was agitated, the water would con- 
tinue to move for some time after the bason 
was left to stand still; or like a pendulum, 
which having been put in motion by the hand, 
continues to make several vibrations without 
any new impulse. When the Aloon is in the 
equator, the tides are equally high in both 
parts of the lunar day, or time of the Moon’s 
revolving, from the meridian to the meridian 
again, which is 24 hours 50 minutes. But as 
the Aloon declines from the equator towards 
either pole, the tides are alternately higher 
and lower at places having north or south 
latitude. For one of the highest elevations, 
which is that under the Moon, follows her 
towards the pole to which she is nearest, and 
the other declines towards the opposite pole; 
each elevation describing parallels as far dis- 
tant from the equator, on opposite sides, as 
the Aloon declines from it to either side ; and 
consequently the parallels described by those 
elevations of the water are twice as many de- 
grees from one another as the Aloon is from 
the equator ; increasing their distance as the 
Moon increases her declination, till it is at the 
greatest, when these parallels are, at a mean 
state, 47 degrees from one another ; and on 
that day tire tides are most unequal in their 
heights. As the Moon returns tow ards the 
equator, the parallels described by the oppo- 
site elevations approach towards each other, 
until the Moon comes to the equator, and 
then they coincide. As the Aloon declines 
towards the opposite pole, at equal distances, 
each elevation describes the same parallel in 
the other part of the lunar day, which its op- 
posite elevation described before. Whilst the 
Moon has north declination, the great tides 
in the northern hemisphere are when she is 
above the horizon ; and the reverse whilst her 
declination is south. 
In open seas the tides rise but to very small 
heights, in proportion to what they do in wider 
mouthed rivers, opening in the "direction of 
the stream of tide. For in channels growing 
narrower gradually, the water is accumulated 
by the opposition' of the contracting bank ; 
like a gentle wind, little felt on an open plain, 
but strong and brisk in a street ; especially if 
Lhe wider end of the street is next the plain, 
and in the way of the wind. 
The tides are so retarded in their passage 
through .different shoals and channels, and 
otherwise so variously affected bv striking 
against capes and headlands, that to different 
places they happen at all distances of the Aloon 
from the meridian, consequently at all hours 
of the lunar day. The tide propagated by r 
the Aloon in the German Ocean, w hen she is 
three hours past the meridian, takes twelve 
hours to come thence to London-bridge, 
where it arrives by the time that a new tide 
is raised in the ocean. There are no tides iit 
lakes, because they are generally so small, 
that when the Moon is vertical she attracts 
every part of them alike; and therefore, by 
rendering all the waters equally light, no 
part of them can be raised higher than another, 
the Mediterranean and Baltic Seas suffer 
very small elevations, because the inlets by 
which they communicate with the ocean are 
so narrow, that they cannot, in so short a 
time, receive or discharge enough to raise or 
sink their surface sensibly. 
Air being lighter than water, gnd the surface 
of the atmosphere being nearer to the Aloon 
than the surface of the sea, it cannot be 
doubted that the Aloon raises much higher 
tides in the air than in the sea. And therefore 
many have wondered why the mercury does 
not sink in the barometer when the Aloon’s 
action on the particles of air makes them 
lighter as she passes over the meridian. But 
We must consider, that as these particles are 
rendered lighter, a greater number of them is 
accumulated, until the deficiency of gravity 
is made up by the height of the column ; anil 
then there is an equilibrium, and consequent- 
ly an equal pressure upon the mercury as be- 
fore ; so that it cannot be effected by the 
aerial tides. It is very probable, however, 
that the stars which are seen through an aerial 
tide of this kind will have their light more re- 
fracted than those which are seen through the 
common depth of the atmosphere ; and this 
may account for the supposed refractions by 
the lunar atmosphere that have been some- 
times observed. 
Z 
Of Comets. Tycho Brahe and Cassini were the 
first among the modern astronomers who as- 
signed these bodies a place in our system ; hut 
they appear’ to be unacquainted' both with 
