long ago as 1591, that the stars may be suns attended perhaps by planets like our earth. This is the most probable first assumption, supported in some degree by very recent spectrum observations, which show the similarity of light derived from many stars with that of the sun. But at the same time we learn by the prism that there are nebulæ and stars in conditions widely different from anything known in our system. In the course of time the analogy may perhaps be restored to comparative completeness by the discovery of many suns in various stages of nebulous condensation. The history of the evolution of our own world may, as it were, be traced back in bodies less developed, or traced forwards in systems more advanced towards the dissipation of energy, and the extinction of life. As in a great workshop, we may perhaps see the material work of Creation as it has variously progressed through thousands of millions of years.

By the careful delineation and classification of the nebulæ and stellar systems, we may hope in time to find some parallel even to that apparently space-filling system of the Milky Way. Michell pointed out that the Pleiades form a remarkable group of worlds, and he thought that it might present an analogy to the sun and its immediate neighbours. The observations of the Herschels and other more recent astronomers, show that we really belong to a vast stratum of worlds of a peculiar split form, including countless myriads of stars of various sizes. The belief in analogy is irresistible, and astronomers have already looked into the depths of space, hoping to find distant nebulous specks which might resemble the supposed form of the Milky Way, and extend our knowledge to a higher order of universes. Such expectations are probably premature, or even unfounded; nevertheless in the forms of the nebula we may find much instruction. The spiral form disclosed in many bodies

by Lori Fosse's triese pe pussesses some analogy to what would happen in a system reviving in a dense retarding melam. Let us once ascertain by the spectroscope that there is a dense envei pe cf gas, and the forms of those bodies are at once brought into harmony with the laws of matter on this globe. Viewing such worlds as we do from a fixed distant point, they appear variously distorted according to the laws of perspective; but when we find in many objects forms which might have proceeded from the same object variously inclined to the line of vision, analogy will aid us in determining the real form. Thus when we see an apparent nebulous ring, we may be unable to decide whether it is really a ring of matter or a spherical shell, of which the obliquely seen edges are aloce apparent. But if elsewhere we discover, as did Lori se another nebula presenting the distinct appearance a ring seen edgeways, we may infer with some probability fm one case to the other. By similar proesses of yarison and analogical reasoning, we may in with much confidence the absolute forms of mass of lestial objectsTM. In speculations concern, other planets and heavers

ysical condition of must often depend We may be

upon analogies of a very sancter. said to know that the men s montains and valleys, plains and ridges, volcanoes and streams of lava, and, in spite of the absence of air and water, the rocky surface of the moon presents so many familiar appearances that we do not hesitate to compare them with the features of our own globe. We infer with high probability that Mars has polar snow and an atmosphere absorbing blue rays like our own; Jupiter undoubtedly possesses a cloudy atmosphere, possibly not unlike a magnified copy of that surrounding the earth, but our tendency to adopt anm Grant's History of Phy omy,' pp. 570-571.

alogies receives a salutary correction in the recently discovered fact that the atmosphere of Uranus contains hydrogen. Philosophers of the highest grade have not stopped at these comparatively safe inferences, but have speculated on the existence of living creatures in other planets. Huyghens remarked that as we infer by analogy from the dissected body of a dog to that of a pig and ox or other animal of the same general form, and as we expect to find the same viscera, the heart, stomach, lungs, intestines, &c., in corresponding positions, so when we notice the similarity of the planets in many respects, we must expect to find them alike in other points". He even enters into an inquiry whether the inhabitants of other planets would possess reason and knowledge of the same sort as ours, concluding in the affirmative. Although the power of intellect might be different, he considers that they would have the same geometry if they had any at all, and that what is true with us would be true with them. As regards the sun, he wisely observes that every conjecture fails. Laplace entertained a strong belief in the existence of inhabitants on other planets. The benign influence of the sun gives birth to animals and plants upon the surface of the earth, and analogy induces us to believe that his rays would tend to have a similar effect elsewhere. It is not probable that matter which is here so fruitful of life, would be sterile upon so great a globe as Jupiter, which, like the earth, has its days and nights and years, and changes which indicate active forces. Man indeed is formed for the temperature and atmosphere in which he lives, and, so far as appears, could not live upon the other planets. But there might be an infinity of organizations relative to the diverse constitution of the bodies of the universe. The most active imagination cann 'Cosmotheoros' (1699), p. 17.

o Ibid p. 36.

not form any idea of such various creatures, but their existence is not unlikely P.

We now know that many metals and other elements never found in organic structures are yet capable of forming compounds, with substances of vegetable or animal origin. It is therefore just possible that at different temperatures creatures formed of different but analogous compounds might exist, but it would seem indispensable that carbon should still form the basis of organic structures; for we have no analogies to lead us to suppose that in the absence of that complex element, life can exist. Could we find globes surrounded by atmospheres resembling our own in temperature and composition, we should be almost forced to believe them inhabited, but the probability of any analogical argument decreases rapidly as the condition of a globe diverges from that of our own. The Cardinal Nicholas de Cusa held long ago that the moon was inhabited, but the absence of any appreciable atmosphere renders the existence of inhabitants highly improbable. Speculations resting upon weak analogies hardly belong to the scope of true science, and can only be tolerated as an antidote to the far worse dogmatism which would assert that the thousand million of persons on earth, or rather a small fraction of them, are the sole objects of care of the Power which designed this limitless Universe.

Failures of Analogy.

So constant is the aid which we derive from the use of analogy in all attempts at discovery or explanation, that it is most important to observe in what cases it may lead us into difficulties. That which we expect by analogy to exist may

P System of the World,' vol. ii. p. 326. Essai Philosophique,' p. 87.

(1) Be found to exist;

(2) May seem not to exist, but nevertheless may really exist;

(3) May actually be non-existent.

In the second case the failure is only apparent, and arises from our obtuseness of perception, the smallness of the phenomenon to be noticed, or the disguised character in which it appears. I have already pointed out that the analogy of sound and light seems to fail because light does not bend round a corner, the fact being that it does so bend in the phenomena of diffraction, which present the effect, however, in such an unexpected and minute form, that even Newton was misled, and turned from the correct hypothesis of undulations which he had partially entertained.

In the third class of cases analogy fails us altogether, and we expect that to exist which really does not exist. Thus we fail to discover the phenomena of polarization in sound travelling through the atmosphere, since air is not capable of any appreciable transverse undulations. These failures of analogy are of peculiar interest, because they make the mind aware of its superior powers. There have been many philosophers who said that we can conceive nothing in the intellect which we have not previously received through the senses. This is true in the sense that we cannot image them to the mind in the concrete form of a shape or a colour; but we can speak of them and reason concerning them; in short, we often know them. in everything but a sensuous manner. Accurate investigation shows that all material substances retard the motion of bodies through them by substracting energy by impact. By the law of continuity we can frame the notion of a vacuous space in which there is no resistance whatever, nor need we stop there; for we have only to proceed by analogy to the case where a medium should