[Sidenote: On the Law of Gravity] 80. The sphere of the earth has gravity which increases in proportion to the lightness of the element which contains it. Fire is light in its sphere and its lightness increases in proportion to the weight of the element which contains it. {174} No primary element has gravity or lightness in its own sphere. 81. The motion made by bodies which possess gravity to the common centre is not produced by the tendency of the body to find this centre, nor is it caused by attraction made by the centre, as by a magnet, drawing the weight towards it. 82. Why does not the weight remain in its place? It does not remain because it has no resistance. And whither will it tend? It will tend to the centre of the earth. And why not along other lines? Because the weight which meets with no resistance will descend by the shortest way to the lowest depth, and the lowest depth is the centre of the earth. And how does a weight find the centre of the earth with such directness? Because it does not proceed at random, wandering by diverse courses. [Sidenote: Phenomena governed by Mechanical Laws] 83. Instrumental science, that is to say, mechanics, is the most noble and most useful of sciences, inasmuch as by means of it all living bodies which have movement act; and this movement has {175} its origin in the centre of gravity which is placed in the middle, dividing unequal weights, and it has dearth and wealth of muscles and lever also and counter-lever. 84. Since these things are far more ancient than letters, it is no wonder if in our day no records exist to tell how these seas filled so many countries. But if some record had existed, conflagrations, floods, wars, changes of tongues and laws have consumed all that is ancient; sufficient for us is the testimony of objects born in the salt waters and found again in the high mountains far off from the seas of those times. [Sidenote: Heat the Vital Principle] 85. Heat causes moisture to move, and cold arrests it; as is seen in a cold country which arrests the motion of the clouds in the air. Where there is life there is heat, where there is vital heat there is movement of moisture. [Sidenote: Against those desiring to correct Nature] 86. The act of cutting out the nostrils of a horse is a piece of ludicrous folly. And the foolish indulge in this practice as though they considered nature had failed to supply necessary wants, and man had therefore to supplement her work. Nature made two apertures in the nose, which each in {176} itself is half as large as the lung pipe whence breath proceeds, and if these apertures did not exist the mouth would abundantly suffice for breathing purposes. And if you said to me, Why has nature thus provided animals with nostrils if respiration through the mouth is sufficient?--I would answer that nostrils are made to be used when the mouth is employed in masticating its food. [Sidenote: Of Trees] 87.