Suppose it were invited, whether the present land had aver been covered by the sea. , If we look at the stratified form of so large a portion of the earth's surface, we cannot but conclude it to be very probable that such land was formed at the bottom of the sea. But the decisive proof is afforded by the shells and corals, or -bodies having the perfect shape of shells and corals, and of other marine exuvite, which are found imbedded in masses of the most solid rock, and often on the tops of very high mountains. This leaves Ito doubt of the formation of the land under the sea, though it does not determine whether the land, since its formation, has been elevated to its present height, or the sea depressed to its present level. The decision of that question requires other facts to be consulted.
IV. The instantia clandestina, which is, as it were, opposed to the preceding, and shows some power or quality just as it is beginning to exist, and in its weakest state, is of ten very useful in the generalization of facts. Bacon also gave to this the fanciful name of instantia crepusculi.
An example of this may be given from hydrostatics. If the suspension of water in ca pillary tubes be inquired into, it becomes very useful to view that effect when it is least, or when the tube ceases to be capillary, and becomes a vessel of a large diameter. The co lumn is then reduced to a slender ring of water which goes all round the vessel, and this, though now so inconsiderable, has the property of being independent of the size of the vessel, so as to be in all cases the same when the materials are the same. As there can be no doubt that this ring proceeds from the attraction of the sides, and of the part immediately above the water, so there can be no doubt that the capillary suspension, in part at least, is derived from the same cause. An effect of the opposite kind takes place when a glass vessel is filled with mercury.
V. Next to these may be placed what are called.instantite nsanipulares, or collective in stances, that is, general facts, or such as comprehend a great number of particular cases. As human knowledge can but seldom reach the most general cause or form, such collective instances are often the utmost extent to which our generalization can be carried. They have great value on this account, as they likewise have on account of the assistance which they give to farther generalization.
Of this we have a remarkable instance in one of the most important steps ever taken in any.part of human knowledge. The laws of Kepler are facts of the kind now treated of, and consist of three general truths, each belonging to the whole planetary system, and it was by means of them that Newton discovered the principle of gravitation. The
first is, that the planets all move in elliptical orbits, having the sun for their common focus; the next, that about this focus the radius vector of each planet describes equal areas in equal times. The third and last, that the squares of the periodic times of the planets are as the cubes of their mean distances from the sun. The knowledge of each of these was the result of much research, and of the comparison of a vast multitude of observations, insomuch that it may be doubted if ever three truths in science were dis covered at the expense of so much labour and patience, or with the exertion of more ingenuity and invention in imagining and combining observations. These discoveries were all made before Bacon wrote, but he is silent concerning them; for the want of ma thematical knowledge concealed from his view some of the most splendid and interesting parts of science.
Astronomy is full of such collective instances, and affords them, indeed, of the second and third order, that is to say, two or three times generalized. The astronomer observes nothing but that a certain luminous disk, or perhaps merely a luminous point, is in a cer tain position, in respect of the planes of the meridian and the horizon, at a certain mo ment of time. By comparing a. number of such observations, he finds that this luminous point moves in a certain plane, with a certain velocity, and performs a revolution in a cer tain time. Thus, the periodic time of a planet is itself a collective fact, or a single fact expressing the result of many hundred observations. This holds with respect to each planet, and with respect to each element, as it is called, of the planet's orbit, every one of which is a general fact, expressing the result of an indefinite number of particulars. This holds still more remarkably of the inferences which extend to the distances of the planet from the earth, or from the sun. The laws of Kepler are therefore collective facts of the second, or even a higher order ; or such as comprehend a great number of general facts, each of which is itself a general fact, including many particulars. It is much to the cre dit of astronomy, that, in all this process, no degree of truth or certainty is sacrificed ; and that the same demonstrative evidence is preserved from the lowest to the highest point. Nothing but the use of mathematical reasoning could secure this advantage to any of the sciences.