ASTRONOMY. 



ty than those of the moon ; and conse- 

 quently will have a less effect in produc- 

 ing any change of its figure. 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 si- 

 tuations. That this doctrine may be still 

 more clearly understood, let it be consi- 

 dered, that though the earth's diameter 

 bears a considerable proportion to the dis- 

 tance of the earth from the moon, yet this 

 diameter is almost nothing, when com- 

 pared to the distance of the earth from 

 the sun. The difference of the sun's at- 

 traction, 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 calculated the effect of 

 the sun's influence in this case, and found 

 that it is about three times less than that 

 of the moon. The action of the sun alone 

 would therefore be sufficient to produce 

 a flux and reflux of the sea ; but the ele- 

 vations and depressions occasioned by 

 this means would be about three times 

 less than those produced by the moon. 

 The tides, then, are not the sole produc- 

 tion of the moon, but of the joint forces 

 of the sun and moon together : or, pro- 

 perly speaking, there are two tides, a solar 

 one and a lunar one ; which have a joint 

 or opposite effect, according to the situa- 

 tion of the bodies which produce them. 

 When the aciions of the sun and moon 

 conspire together, as at the time of new 

 and full moon, the flux and reflux become 

 more considerable ; and in this case they 

 are called the spring tides. But when 

 one tends to elevate the waters, whilst 

 the other depresses them, as at the moon's 

 first and third quarters, the effect will be 

 exactly the contrary ; the flux and reflux, 

 instead of being augmented, as before, 

 will now be diminished ; and they are 

 then called the neap tides. But as this is 

 a matter of some importance, it may be 

 worth while to enter into a more minute 

 explanation of it. 



For this purpose, let S (ftg. 5) repre- 

 sent the sun, ZHNK the earth, and F C 

 the moon, at her full and change. Then, 

 because the sun, S, and the moon, (J, are 

 nearly in the same right line with the centre 

 of the earth, O, their actions will conspire 

 together, and raise the water about 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 



VOL. 11. 



been shewn, that when the ocean is ele- 

 vated at the zenith, Z, it is also elevated 

 at the opposite point, or nadir, N, at the 

 same time ; and therefore, in this situa- 

 tion of the sun and moon, the tides will 

 be augmented. Again, whilst the full 

 moon, F, raises the waters at N and Z, di- 

 rectly under and opposite to her, the sun 

 S, acting in the same right line, will also 

 raise the waters at the same points, Z and 

 N, directly under and opposite to him ; 

 and therefore, in this situation also, the 

 tides will be augmented, their joint ef- 

 fect being nearly the same at the change 

 as at the full; and in both cases they oc- 

 casion what are called the spring tides. 

 Pursuing the illustration in the same way, 

 let now F and T (fig. 6.) be the moon in 

 her first and third quarters, and the rest 

 as before. Then, since the sun and moon 

 act in the right lines S H and F T, which 

 are nearly perpendicular to each other, 

 their forces will tend to produce contrary 

 effects ; because the one raises the waters 

 in that part where the other depresses 

 them. The sun's attraction at R and H 

 will diminish the effect of the moon's 

 attraction at Z and N ; so that the wa- 

 ters will rise a little at the points un- 

 der and opposite to the sun, and fall as 

 much at the points under and opposite to 

 the moon ; and of course the lunar tides 

 will be diminished in those parts. This 

 respects the moon only in her first quar- 

 ter, at F ; but the same reasoning will 

 evidently hold, when applied to the moon, 

 in her third quarter, at T ; for as the sun. 

 and moon still act in lines which are per- 

 pendicular to each other, they must pro- 

 duce the same diminution as before ; and 

 in both these cases they occasion what are 

 called the neap tides. But it must be ob- 

 served, that neither the spring nor neap 

 tides happen when the sun and moon have 

 the precise situations here mentioned ; 

 because, in this case, as in others of a si- 

 milar kind, the actions do not produce 

 the greatest effect when they are the 

 strongest, but some time afterwards. The 

 effects of the disturbing forces of the sun 

 and moon depend, likewise, upon their 

 respective distances from the earth, as 

 well as upon their particular situations. 

 For the less the distances are, the great- 

 er will be their effects ; and, consequent- 

 ly, in winter, when the sun is nearer to 

 the earth, the spring tides will be greater 

 than in summer, when he is farther off; 

 and the neap tides, on that account, will 

 be less. For a like reason, as the moon 

 moves in an elliptical orbit round the 

 earth, and is nearer to us at some times 



