I Have often tryed to have a proper idea of vast space—great numbers—enormous size and such subjects, and as you may suppose, without success. But though I fail in getting a competent idea, I sometimes make an approach towards it, which is better than nothing.

The solar system is one of these sublime subjects in the consideration of which I have frequently been lost. I never attempted to conceive the size of the sun, or the distance of saturn; the impossibility instantly repels the most daring imagination. No, all that I have attempted is to have a just idea of the proportion (upon any scale) that the sun and planets bear to each other in respect to size and distance. At first sight, this seems easily done—Draw some concentric circles on a sheet of paper, make the sun the centre, and place the planets round in their order.—Or if you would have an idea of their motion as well, look at an orrery. But a little examination will convince you that this is doing nothing towards having an idea of their size and distance in proportion to each other, which is the point sought. Nay, it is worse than nothing, for it imposes a falsity as a reality. Imagination by itself can do a great deal, if assisted it can do more, but if perverted, nothing. Let us try to assist the imagination then.

If the sun be only a million times bigger than the earth, (exactness is of no consequence to my argument, so that I am within the truth) it is plain that I cannot make two circles upon a sheet of paper (without considering any thing about distance) that can bear this proportion to each other; and if this cannot be done for the earth, much less can it for other planets and moons where the disproportion is greater. Let us take the floor of a large room—on this make a circle of two feet diameter for the sun—the size of the earth will be about a large pin’s head. The distance of the sun from the earth is about eighty of the sun’s diameters; if so, there must be a circle of three hundred and twenty feet diameter for the earth’s orbit, which no room, nor indeed any other building, will contain. Let us try a field——here we may put our sun and draw the earth’s orbit round. If we stand in the center (which we should do) the earth is too small to be seen. These difficulties occurring so soon, how will they increase when we take in the superior planets? The ingenious Ferguson has endeavoured to assist our imagination by supposing St. Paul’s dome, in diameter one hundred and forty-five feet, to be the sun——upon this scale, Mercury is between nine and ten inches, and placed at the Tower; Venus near eighteen, at St. James’s Palace; the Earth eighteen, at Marybone; Mars ten, at Kensington; Jupiter fifteen feet, at Hampton-Court; and Saturn eleven feet and half, at Cliffden. Let us be on the top of the dome, and look for the planets where he has placed them. Do you think we could see any thing of Jupiter and Saturn? to say nothing of their moons—or that we could conceive properly the difference between four miles and twenty, when seen on a line? the four may be two, or one mile; and the twenty may be ten, or thirty, for ought we can judge by the appearance. All that we get by this is the knowing that a sheet of paper or an orrery give us wrong ideas, and that we cannot by any contrivance put the size and distance of the planets upon a proportionable scale, so as to take in the whole with our eye or understanding.

We are as much at a loss to comprehend the slowness of their motion—I have not mistaken—I mean slowness.—A circuit which is six or twelve months or twice as many years performing, is slow almost beyond conception; and yet this motion is called whirling—as if the planets went round their orbits like a top! Though quick and slow are comparative terms, we have ideas of each arising from the medium of the two, from observation, and common application, that do not stand in need of any comparison to be understood. The motion of a flea is quick; of a snail, slow; and the common walk of a man is neither quick nor slow. Let us imagine an elephant to walk, and a flea to hop the same distance in the same time—would you hesitate to say that the motion of the one was slow, and the other quick? In short, swiftness or slowness does not depend upon the absolute quantity of ground the animal passes in a certain time, but upon the relative quantity to its own size. The earth is about eight minutes in moving the space of one diameter, therefore its absolute motion is slow—it is twenty-four hours making one revolution round its axis, which gives no idea of velocity. It is certain that if we were placed very near the earth (unaffected by its attraction) there would appear an exceeding quick change of surface—and so would the motion of a snail appear to an animalcule. The quantity of space when compared to any we can move in the same time is vast, and the motion quick, but when considered as belonging to a body of the size of a world, the motion is slow. Suppose a common globe was turned round once in twenty-four hours—imagine an animal as much inferior to it in size as we are to the earth, placed as I conceived the human spectator placed to view the earth—would the apprehension of this Being induce you to call a single revolution in twenty-four hours, whirling? Would not you say that though the surface passed quick in review before him, yet that the absolute motion of the whole was exceedingly slow. Perhaps it is our measuring this motion by miles that makes us fancy that it is quick, which is much like taking the height of a mountain in hairs-breadths. When we are told that Saturn moves in his orbit more than twenty-two thousand miles in an hour, we conceive the velocity to be great; but when we find that he is more than three hours moving his own diameter, we must then think it as it really is, slow. Bishop Wilkins is the only writer I have met with who considers the motion of the heavenly bodies as I do, and I am rather proud of having my opinion supported by so great a man.

There is another circumstance which prevents the solar system, as commonly delineated, from bearing a true resemblance to the apparent position and motion of the planets. It is always drawn in plan instead of section, whereas the appearance of the orbits of the heavenly bodies is always in section and never can be in plan. This difference is not, as far as I know, noticed in any account of the solar system; and yet if it be not attended to, it is impossible to prove the truth of the system by the apparent paths of the planets. This will be best understood by considering the inferior ones. Mercury and Venus remove to a certain distance from the sun, and then, after seeming at rest, return in nearly the same line and remove to the same distance on the other side, where the same thing is repeated. This to the eye is not a revolution in plan, but a revolution in section—and this might be explained by a draught which should always accompany the common delineation of the planetary orbits.