each other. If the relative magnitudes or the relative distances of these bodies had been different from what they are, the tides must likewise have been different. There appears, for instance, no necessary connection between the force of gravity upon the surface of a planet, and the magnitude and distance of a secondary planet which revolves about it. Now, had the moon been much nearer the earth than it is, the tides produced in the ocean might have been so great, as to have overwhelmed the dry land and all its inhabitants. On the contrary, had the moon been much more distant, the tides would have been too small to be essentially beneficial. Neither, again, does there appear connection between the distance and mass of a secondary planet, and the time in which the primary planet revolves about its own axis. The earth might have performed its diurnal revolution in the same time that the planet Jupiter does, in about twelve hours: and still might have assumed a form not form not very different from that which it now possesses.

any

But the tides of its ocean would then have reciprocated with nearly double their present velocity. It would have been, perhaps, impossible to navigate rivers, or to approach those parts of the ocean where local circumstances increase the motions of the tides.

Since then, the magnitudes and relative position of the sun, the moon, and the earth are such as to produce effects upon the ocean acknowledged to be beneficial, when contrary effects might have been produced, we conclude that the effects were foreseen: and that the system affords in this point of view a striking instance of providential adjustment.

8. But, it may be said, if analogy may be relied upon, we have at least some reason to conclude that the secondary planets may be bodies constituted in the same manner as this earth, and their respective primary planets. They may be solid bodies partly covered with a circumambient fluid: and if so, the fluctuations, arising from the difference of the attraction of the primary upon different parts of their surface, must be far greater than the undulations which they could excite upon the fluids of the larger body. For instance, if the moon were composed of parts similar to that of the earth, the force of the earth to disturb its waters would be full ten times as great as that which the moon exerts upon the waters of our ocean.h How can this danger be obviated? By a relation the most wonderful, may we not say the most unexpected, that can be conceived.

Newton, Principia, vol. III. Prop. 38.

i

All the secondary planets, upon which observations have successfully been made to establish the fact of their rotation, are found to revolve upon their own axes, in the very same time in which they perform their revolution about their respective primaries. The consequence of this correspondence is, that they always turn the same part of their surface towards the primary planet; and therefore that the general form of the fluids upon their surfaces will not be periodically altered. The secondary planets will be elongated in the direction of the diameter which passes through the center of the primary; but no succession of changes will take place similar to the reciprocations of the tides upon the earth. It is said, indeed, that this equality would arise simply from physical causes. That, if the angular

motion of a satellite about its axis had been very nearly equal to its mean angular motion in its path about the primary, the attraction of its primary might have made the two motions coincide. And it is proved that, after the secondaries had acquired the form which, as fluid bodies, they would assume by the attraction of their primaries, on the supposition

i The Moon; all the satellites of Jupiter; and one of Saturn. See Newton, Principia, vol. III. Prop. 38. Dan. Bernouilli, Traité sur le flux et reflux de la mer. ch. iii. §. 5. Brinkley's Astron. §. 125, 130.

that the same face is turned towards them, the large secular variations which effect the revolutions of the primary bodies would be communicated to the secondaries; so that the equality between the time of their revolution and the time of their rotation would still subsistk. But, allowing this to be the case, the only question would be, with what

degree of accuracy a particular design was originally effected; whether it were accomplished entirely, or only partially by the adjustment of the initial angular velocity. All the primary planets revolve about their axes; and no general proportion is observed between their diurnal and annual revolutions.l Why is this apparent absence of order suddenly changed in all the secondaries? Of all possible periods which could have been selected for a satellite, that which would permit the attraction of its primary to reduce its angular motion about its axis to an equality with the motion in its orbit, lies within very small limits. Had the initial angular velocity been greater or less than those contained within such limits, the observed equality never could have taken

* Mécanique Celeste, Part I. Liv. v. ch. ii.

1 The two large planets Saturn and Jupiter revolve about their axes nearly in the same time, 12 hours, although their periods are very different. And all the other planets inferior to them probably revolve in about 24 hours.

place. Yet this nice adjustment is found, not in one or two satellites only, but in many; and probably extends to all the secondary bodies of the system.

Shall we then consider it to be improbable in the highest degree," that the Creator of the universe should give such a degree of motion, as would obviate a great inconvenience, if an analogy not very forced may be relied upon? Shall we not rather recognize, in this adaptation, the proofs of wise and beneficent design?

9. Another circumstance, which has always been considered as affording a strong presumption of contrivance and design in the constitution of the system which we inhabit, is the fact, that the central body only is luminous. There seems to be no intelligible reason why, if a number of bodies be made to revolve about their common center of gravity, the largest alone should be capable of illuminating and warming the rest. But it is evident that the present frame of the solar system is precisely fitted for diffusing light and heat equably throughout the system; and that the same purpose could not have been effected, had any

m II serait contre toute vraisemblance de supposer qu'à l'origine ces deux mouvemens ont ete parfaitement egaux. Exposit. du Systeme du Monde, Liv. Mr. ch. xiv.