have on the motions of the planetary bodies, we may venture from what we know of matter, to assert, that it is, of itself, inactive: and therefore, when in motion, we cannot help concluding that some power has put it in motion. We have also a right to conclude from what we perceive of moving bodies, that a body propelled forward would proceed in a straight line, unless there existed some power to divert it from that line; and we may argue that, if there were no resisting force,-nothing to stop it, a body in motion would continue to move for ever in that line. But bodies moving just above the

surface of the Earth meet with two obstacles to their progress; one, the resistance of the air; the other and most important is gravity, or the attraction existing in the Earth.

The Planets move around the Sun in orbits nearly approaching to circles; and since these revolutions have continued the same, and with the same degrees of velocity, from the earliest observation, have we not reason for concluding that they have so moved ever since their creation? As matter is of itself inactive, and as the planets are solid bodies, it is reasonable to conclude that a propelling force was given to them. If the force that propels forward had alone been given to them, they would have moved in straight lines; but as they deviate from such lines, and move nearly in circles, some cause for this deviation must exist; and we may conclude that, as they move round the Sun, the cause of their so moving exists in the Sun, and that the power existing in it is attraction or gravity.

Hence, it follows that all the planetary bodies have two powers exerted on them; one, that propelling force which would have sent them forward in straight lines, if

the other-Gravity-did not cause them to deviate from the straight line; and these two powers are so precisely balanced, so nicely proportioned to each other, as to keep the planets and comets in their orbits round the Sun.

Now as gravity exists in a body, not in proportion to its bulk, but to its solidity, we should argue that the Sun must be much larger or almost infinitely denser than the bodies moving around it by its attraction. The Sun is computed to be 111 times the diameter of the Earth, 1,370,000 times its bulk, and 227,000 times its weight.*

* The term density has reference to the closeness or compactness of the materials of which a body is composed. If a pound of feathers or of wool was packed in the compass of a sphere one foot in diameter, the compactness of the feathers or wool would be very different-the density of the mass would be much lessthan if the same quantity were packed in a sphere six inches in diameter..

The density of the Sun and planets is of course altogether a matter of theory, dependent on analogical reasoning, founded on their relative dimensions and masses of matter, and the laws of gravitating bodies. We know that the Moon moves about the Earth-is kept in her orbit by the Earth's known attraction-and that all the peculiarities of her motion are consonant therewith. We therefore assume the right to conclude, that the moons of the other planets are kept in their orbits by the attraction of the planets about which they revolve: The nearest of Jupiter's moons is about the same distance from him that our Moon is from the Earth. If these moons moved with the same velocity, we should say that Jupiter and the Earth possess the force of attraction in the same degree, provided the mass of Jupiter's satellite equalled that of our Moon; but the satellite of Jupiter moves about 16 times faster than our Moon. Now the bulk of Jupiter is 1400 times greater than that of the Earth; therefore his force is not in proportion to his size; for if it were, his first or nearest satellite, if equal in matter to the Moon, would move nearly 88 times

How absurd therefore would it be to suppose that the Sun moves round the Earth; it would not be less absurd than to expect a mill-stone to move round a pea, merely by the force of attraction.

As the Moon accompanies the Earth round the Sun, and the Satellites of the other planets accompany them, it must be by the force of attraction existing in the primary planets for their satellites: so that all the bodies of the Solar system possess it. This power must always exist-every moment of time; for if it were to relax for one moment, the harmony of the system would be de

swifter than it does. It is calculated that the substance of which Jupiter is composed is four times lighter and less dense than that of the Earth. Pursuing the calculation, the mathematician has determined that Mercury is above 9 times, Venus 5 times, and the Earth 4 times more dense than the Sun, whose density is only 1 and 1-8th greater than that of water. Therefore, it is not by his density, but by the force of the attraction of his prodigious quantity of matter that the Sun keeps the whole planetary system in harmonious revolution. The quantity of matter in the Sun is calculated to exceed that of all the planets, if they were in one mass, 530 times.

Taking water as the standard of comparison, it is calculated that the density of the

stroyed, and the consequence would be that the projectile force would instantly carry off the planets in straight lines.

The existence of Attraction in all the planetary bodies is not ideal; for the observations of many acute observers of the planetary motions concur in shewing, that when in their nearest approach towards each other, they disturb one another's motions in a slight degree. Indeed for some time before the discovery of the Georgium Sidus, some irregularities were observed in the motions of Jupiter and Saturn, which could only be accounted for by supposing that there existed yet another planet beyond them.-The Moon, by its attractive power, fluences the tides, and therefore must attract the Earth; and as the Sun attracts all the Planets, so the Planets. attract the Sun, and cause it to move around a central point, termed the centre of attraction of the system; which thus is kept in the most harmonious revolution.

in

Here, the observation with which I began, forcibly recurs. This subject is of the utmost dignity and interest. What an inexhaustible field for reflection does it present. When such effects are produced, we cannot mistake the cause. In the creation of these orbs, and in their harmonious movements, we cannot fail to see the hand of an Artificer whose power and intelligence are infinite.

Of the nature of Gravity or Attraction, or rather wherein it consists, we really know nothing. It is a power that, while we perceive and must acknowledge its effects, eludes our senses most completely. It is that power which keeps every thing at rest on this revolving ball, until some force shall give it motion. Every thing moving on the surface of the Earth is influenced by it. The mass that descends from the mountain top, and the

apple that falls from the tree, are alike under its dominion. Even man himself owes to this power his firm footing on the ground he treads; or how would it be possible for us and our antipodes, or those who live on the opposite side of the Earth, to tread firmly each with his feet towards the centre of the Earth and his head the contrary way? How mysterious then is this power! Of so subtle a nature is it that we cannot define it, but are left to the necessity of supposing it to be a law impressed upon all matter by the immediate agency of the Creator: but that it is a law existing under peculiar modifications of its exertion, we are absolutely certain; for if a body, in a complete state of rest, be let fall towards the Earth, experiment proves that it will fall 16 feet in one second of time; but the same rule does not hold for the next second, in which it will fall 48 feet;-in the 3rd second it will fall 80 feet, and in the 4th second 112 feet. Now in this there is a direct rule. In the first second it fell 16 feet; in the next, 48; together 64 feet in 2 seconds. Let us multiply the 2 seconds by its own number, and the result is the square of that number, or 4; by this number, let us multiply 16, the number of feet it fell in the first second, and the result is 64; being the number of feet it fell in 2 seconds. Proceeding thus, we should find the same rule applicable to the number of feet it fell in the 3rd and 4th seconds and so on. Whence this rule has been found to hold in all cases, namely, the spaces passed through by bodies falling from a state of rest, increase, as the squares of the times they are falling increase.*

* Thus, in the two first seconds, a body falls 64 feet, in the 3rd second 80 feet, in 3 seconds, therefore, 144 feet. Let us prove the rule by trying whether this be true according to it. We wish