arrives at the conclusion that the power of accommodation which vegetables possess in this respect is far from being such as either to leave the existence of the periodical constitution doubtful, or to entitle us to suppose that the day might be considerably lengthened or shortened without injury to the vegetable kingdom.'

But we come next to a feature of very great importance in the economy of Mars, the attraction of gravity at his surface. The reader is aware that the attraction of gravity at the earth's surface depends on the quantity of matter in the earth, and also on the size of the earth. If the earth contained as much matter as at present, but had a diameter only half as great as at present, gravity would be four times as great as it is now; in other words, everything on the earth would weigh four times as much. But if the earth were of the same size as at present, but contained more or less matter, the attraction of gravity would be correspondingly increased or diminished. Now, in the case of Mars we find a very much smaller quantity of matter, as I mentioned in my last; and if we only considered this difference, we should infer that gravity was only one-ninth as great at the surface of Mars as at the earth's surface. But the diameter of Mars is less than the earth's in the proportion of about 11 to 20; and this tends to increase gravity in the proportion of 20 times 20 to 11 times 11-that is, as 400 to 121, or about 10 to 3. Now, if we first decrease our terrestrial gravity to one-ninth its value, and then increase the result as 10

to 3, we get finally a decrease in the proportion of 10 to 27. This shows that a body which would weigh 27 pounds on the earth would, if removed to Mars, weigh only 10 pounds; using the word 'weigh' to mean the actual pressure downwards, for of course in the ordinary way of measuring weights by a balance there would be no difference.

Now, this difference in the downward pressure of all objects on Mars, as compared with terrestrial objects of equal mass, would produce results of a very mischievous nature if it were suddenly to be extended to our earth. In the first place, the mere change in the weight of all objects, including the bodies and limbs of animals, would lead to a variety of unpleasant results. Whewell remarks that in such a case 6 we should discover the want of the usual force of gravity by the instability of all about us. Things would not lie where we placed them, but would slide away with the slightest push. We should have a difficulty in standing or walking, something like what we have on shipboard when the deck is inclined; and we should stagger helplessly through an atmosphere thinner than that which oppresses the respiration of the traveller on the tops of the highest mountains.' And he very well notes that all this shows the real importance of those dark and unknown central portions of the earth which we are apt to regard as 'deposits of useless lumber without effect or purpose. We feel their influence on every step we take and on every breath we draw; and the powers we

possess, and the comforts we enjoy, would be unprofitable to us if they had not been prepared with reference to those as well as to the near and visible portions of the earth's mass.'

Another instance of the importance of the actual value of the force of gravity is found in the correspondence between the force with which the sap of plants is impelled upwards, and the downward action of gravity restraining this upflow. It may, perhaps, be thought at first by many readers that the upward force producing the flow of sap is but slight, since this flow is so gentle a process; but it will suffice to mention the experiments of Hales to show that this is not the case. He found, for instance, that a vine in what is called the bleeding season can push up its sap in a glass tube to a height of twenty-one feet above the stump of an amputated branch.' It is clear that any considerable change in the force of gravity would be most injurious to plant life. An increase of gravity would cause the activity of the vegetable circulation to be greatly reduced; an increase of gravity would unduly hasten the rising of the sap, and probably hurry and overload the leaves and other organs, so as to interfere with their due operation.'

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Another illustration, and a very beautiful one, is pointed out by Whewell in the positions of flowers. Some flowers grow with the hollow of their cup upwards; others "hang the pensive head" and turn the opening downwards.' It is obvious that an increase of gravity would

force the upright plants to hang their heads, while a decrease to the value of gravity which actually exists in Mars would cause the drooping heads to stand erect. But it has been shown by Linnæus that on the position of the heads of flowers, combined with the greater or less length of the pistil and stamens, depends the fertility of the plant. So that, as Whewell remarks, the whole mass of the earth, from pole to pole, and from circumference to centre, is employed in keeping a snowdrop in the position most suited to the promotion of its vegetable health.'

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But the most important effect of all is that which would be produced on the atmosphere. If gravity were reduced to precisely the value it has in Mars, our air would immediately be released from more than half the coercive force now drawing it downwards to the earth. It would still be drawn downwards, but so much less that the density of the air at the sea-level would be reduced in the same degree as gravity—that is, in the proportion of 10 to 27. The mercurial barometer would, in fact, stand at about 11 inches instead of 30. This corresponds to the effect which takes place in an elevation of about five miles. Now, there are no creatures but certain races of birds which exist on our earth at this enormous elevation, and probably very few races of terrestrial animals could survive the change to so rare an atmosphere.

Now we are indeed here touching upon a somewhat doubtful feature of the condition of Mars, whereas I have set myself as a law in the present essay the avoidance of

all doubtful points. We do not know whether there is more or less air around Mars than around our earth; but nevertheless we can be quite certain that in one way or another a state of things must exist which would be very unfavourable to the creatures living on our earth. If on the one hand; the quantity of air is so much greater in Mars that the actual density of the air at the sea-level is the same as with us, then this air, when moving in winds and storms, must be much more effective in overthrowing objects like our terrestrial animals and vegetables, simply because they are so much less strongly kept in their place, in consequence of the feeble gravity of Mars. Besides, if the air is no denser on Mars than with us, there must prevail an intense cold, in consequence of the greater distance of Mars from the sun; and if we have a denser air, the Martial hurricanes become still more destructive. the other hand, if the Martial air is rarer than ours, the cold is still more intense, and thus the condition of Mars is seen to be on this account altogether unfit for such creatures as exist upon our earth.

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We see, then, that for many distinct reasons Mars cannot be the abode of living creatures resembling those with which we are familiar. And we learn to recognise the loving care with which the requirements of terrestrial creatures have been adapted to the circumstances under which they subsist, when we note that even in Mars, the planet which, perhaps, on the whole most nearly resembles our earth, all forms of terrestrial life would quickly perish.