EARTH, in ancient philosophy, one of the elements, the substance of which this globe is composed. To ascertain the density of that substance many experiments have been made; but perhaps none more ingenious than those of Mr Cavendish, which are detailed at full length in Part II. of the Transactions of the Royal Society of London for 1798. They were projected by the late Rev. John Michell, F. R. S. but he did not live to
carry then into effect. After his death the apparatus came to the Rev. F. J. H. Wollaston, Jacksonian Professor at Cambridge, who transferred them to Mr Cavendish. The apparatus contrived for making sensible the attraction of small quantities of matter, and which has been improved by Mr Cavendish, is very simple: it consists of a wooden arm 6 feet long, suspended by the middle in an horizontal position by a slender wire 40 inches long; to each extremity is hung a leaden ball about two inches in diameter; and the whole is inclosed in a wooden case to defend it from the wind.
As no more force is required to turn this balance on
its centre than is necessary to twist the slender suspending wire, the smallest degree of attraction of a leaden weight or weights, a few (eight) inches in diameter, brought near to the small suspended ball or balls of the balance, will be sufficient to move it sensibly aside.
To determine from hence the density of the earth, all that is necessary is, to ascertain what force is required to draw the arm aside through a given space, and then to have recourse to calculation.
To prevent any disturbance from currents that might be produced within the box that contained the balance, by even the difference of temperature that might be occasioned by heat being communicated by the bodies of the experimenters to one side of it more than another, it was supported in the middle of a clove room; the operators, from adjoining apartments, viewed the operation through holes in the wall by means of telescopes; and the apparatus had a strong light thrown upon its two ends (an opening being left at each end of the box for the purpose) by means of two lamps, also in the adjoining apartments, the rays from which were likewise made to pass through the holes formed in the wall.
The two large balls were suspended from a beam near the ceiling, which could be moved in an horizontal direction, by means of a string and pulley, so as to be brought near to the small balls of the balance, or made to recede again, without requiring any person to be in the room.
From this description it will be easily seen, that on the two large balls being brought near to the two small ones, but on opposite sides of each, that their forces may not counteract each other—the small suspending wire of the balance must be twisted by the movements of the arms, occasioned by attraction, which carries the small towards the large balls; and that the wire, endeavouring to untwist itself, will again in its turn carry the small balls away from the large ones. Vibrations are thus occasioned, which would continue a long time before the small balls would settle between the first point of rest and the large balls: but it is not necessary to wait for this; an ivory seal at each end of the balance enables the experimenters, by means of their telescopes, to see the two extreme divisions to which the small balls move in their vibrations, and thus to determine the middle point. The time necessary for each vibration is also noticed.
A full account of these experiments, and of the calculations founded on them, would be little interesting to the great majority of our readers. We shall therefore only mention the result. By a mean of the experiments the density of the earth comes out 5.48 times greater than that of water.
By the experiments made by Dr Maskelyne on the attraction of the hill Schehallien, the density of the earth was computed to be only 4.5 times that of water. The difference of result, therefore, is almost one-fifth, which no doubt must lessen our confidence in either set of experiments, or in the principle on which they were devised.
Earth-Worm (see LUMBRICUS, Encycl.) is an animal which occasions such destruction in gardens, by gnawing the tender roots of shrubs and plants, that various methods have been proposed for remedying this evil. One of the latest, and that which promises to prove the most successful, is given by M. SOCOLOFF in
the fifth volume of the New Transactions of the Imperial Academy of Sciences at Peterburgh. As the destructive power of quicklime, heightened by a fixed alkali, which corrodes or dissolves all the tender parts of animals, has been long known, it occurred to our author that this mixture would be the best means for accomplishing the object which he had in view. He therefore took three parts of quick-lime, newly made, and two parts of a saturated solution of fixed alkali in water, and thence obtained a somewhat milky liquor sufficiently caustic, highly hostile and poisonous to earthworms and other small animals; for as soon as it touched any part of their bodies, it occasioned in them violent symptoms of great uneasiness. If this liquor be poured into those holes in which the earthworms reside under ground, they immediately throw themselves out as if driven by some force; and, after various contortions, either languish or die. If the leaves of plants or fruit-trees, frequented by the voracious caterpillars, which are so destructive to them, be sprinkled over with this liquor, these insects suddenly contract their bodies and drop to the ground. For though Nature has defended them tolerably well by their hairy skins from any thing that might injure their delicate bodies, yet as soon as they touch with their feet or mouths leaves which have been moistened by this liquor, they become as if it were stupified, instantly contract themselves, and fall down.
With regard to plants or corn, these sustain no injury from the liquor, because it has no power over the productions of the vegetable kingdom, as our author has fully learned from experience; or if any hurt is to be suspected, all the danger will be removed by the first shower that falls. This liquor may be procured in abundance in every place where lime is burnt. If the lime be fresh, one part of it infused into about seventy parts of common water will produce real lime water. The want of the fixed alkali may be supplied by boiling wood-ashes in water, and thickening the ley by evaporation.
This liquor might be employed also to kill bugs and other domestic insects; but on account of its strong lixivious smell, M. Socoloff thinks it could not be used with safety in houses that are inhabited. Nothing, however, more speedily or more effectually destroys bugs, as our author says he has repeatedly experienced, than the oily pickle that remains in casks in which salted herrings have been packed.