P L A
feme machine may be fo contrived, as to reprefeiit one fyftem after another, bv Shifting and adjusting the magnitudes and di- stances of the balls. In this manner, the general folar iylrem may be firft exhibited ; then that of the Moon and Earth round the Sun ; afterwards that of Jupiter and its fatelhtes : and, laftly, that of Saturn and its fatelhtes. We have already taken notice of fome of the phenomena explained by the general Planetarium, as well as of that con- trived to exhibit the revolution of the Earth and Moon round the Sun. By thefe is likewife plainly {hewn the caufes of eclipfes, as well as of the apparent ftationary and retrograde motions of the planets.
By the Planetarium adapted to the fyftem of Jupiter, and its fatelhtes, are exhibited the immerfion or entrance of a fatelhte into the fhadow of Jupiter; its emerfion out of the fhadow; when thefe are vifible; when a fatellite is hidden by the body of Jupiter, before it comes into, or after it is gone out of the fhadow ; when a fatellite may be fecn to crofs the body of Jupiter; when a fatellite makes a folar eclipfe in Jupiter; and when Satellites eclipfe one another.
VLt^iT (Suppl.)— ■Propagation o/Plants. Tho' the moft na- tural, as well as mod univerfal method of propagating Plants, is that Of fowing the feeds of each kind in a proper foil ; yet fuch is the prolific power of nature, that in fome inftances, like the polype-animal, they may be propagated by fets, pieces, flips, or cuttings taken from the parent tree, or Plant, and fet in fuch a foil as they are known to delight in. The whole family of willows are propagated in this manner with the greateft eafe. Thofe forts which grow to be large trees, and are cultivated for timber, are generally planted Irom fets of about feven or eight feet long, Sharpened at their larger ends, which is thruft into the ground by the fides of ditches and banks where the ground is moift; in which places they make a confiderable progrefs, and are a great improvement to fuch eftates ; becaufe their tops will be fit to lop every fourth or fifth year. The fallows are planted of kflcr fets, only three feet long, thruft two feet into the ground, and one foot above it. The rows ihould be three feet alunder, and the fets in each row eighteen inches ; obferving always to place the rows Hoping, , in the fame manner as the ground. The beft feafon for planting thefe cuttings is in February ; and if the foil be good, they will produce a great crop of Shoots every year, which will produce, at a moderate computation ten pounds per acre. See Miller's Gard. Diet, in voc. Salix. Befides the willow-kind, the plane-tree, mint, &c. may be propagated in this manner ; only it ought to be remarked, that in providing the flips, fprigs, or cuttings, fuch branches as have knots or joints, mould be cut off two or three inches beneath them ; and that fmall top fprigs of two or three years growth, are the beft for this operation. Ruft. Diet, in voc. Cuttings.
Another way of propagating Plants, is by parting or dividing their roots, each part of which will, by proper management, thrive and fend out frefh roots, which may be Separated in the fame manner. See the article Root, Cycl. and Suppl. The manner of propagating Plants, by laying the tops of branches in the ground, is defcribed under the head Layer, Cycl. and Suppl. And the common way by fowing of the feeds, is Sufficiently explained under the articles Nursery, Sowing, Seed, &c. Cycl. or Suppl.
Sy/lems of Plants. Ray's Syftcm of Plants is delivered in the Cyclopaedia under this head Plant. The fyftem of Lin- naeus is to be found in the Appendix, under the head Fruc- tification ; and the Syftem of Tournefort in the Ap- pendix under the head Petal.
Vegetation and csconomy o/Plants. The theory of the growth and ceconomy of Plants has received great improvmencts from the experiments of the ingenious Dr. Hales, in his vegetable Statics.
It appears from a great number of experiments made by that gentleman, that Plants imbibe and perfpire moifture very co- piously.
As to the quantity of moifture perfpired by Plants and trees, there appears to be a very confiderable difference in different vegetables. ' ■ The lemon-tree, which is an ever-green, per- fpires much lefs than the fun-flower, or than the vine, or the apple-tree, whofe leaves fall off in winter. Thus, from e- qual areas, the lemon-tree was found to perfpire only ^ ¥ of an inch in height, during twelve hours of the day in the month of July ; the apple-tree tJj ; a cabbage ^,; a vine T £ T ; and a fun-flower T J T in a day and night. Hale's Veget. Stat. Vol. I. p. 20, feq. . ^
In order to try what moifture Plants would imbibe, the Dr. cut off Several branches of apple-trees, pear, cherry, and apricot-trees ; and having immerfed them into a known quantity of water, he found fome of them imbibed 1 5 ounces, fome 20 ounces, and others 25 or 30 ounces in 12 hours day more or lefs, according to the quantity of leaves they had ; and and yet when he weighed them at night, they were lighter than in the morning, which undoubtedly was owing to their perfpiration. The quantity imbibed decreafed very much every day, the fap veflels being probably Shrunk at the tranf- verfe cut, and too much Saturated with water, to let any more pafs ; fo that ufually in four or five days the leaves faded avid withered much. Id. ibid. p. 29.
P L A
The Doctor made the fame experiments with elm -I randies, oak, ofier, willow, fallow, afpen, curran, goofberry, and pbil- bert branches; but none of thefe imbibed fo much as the foregoing, and feveral foitsof ever-greens very much lefs. Id. ibid.
He alfo made feveral experiments to afcertain the force With which Plants imbibe moifture. This he did by putting the flump of a branch into one end of a glafs tube, cement- ing it fait ; and then immerfmg the other end of the tube, after being firft filled with water, into a ciftcrn of mercury ; on which removing his finger that flopped up the lower end of the tube, the flump imbibed the water with fo much vi- gour, that in fix minutes time the mercury was raifed eight inches in the tube. It is obfervable, that in all the experi- ments of this fort, the mercury rofe highefl, when the fun was very clear and warm ; and that it fubfided in the night- time, and rofe again the next day, as the fun fhone upon the branch ; only it was neceffary to fill the tube with water ; o- therwife it would not rife at all. It is alfo remarkable, that the top or fmall end of a branch imbibed the water as well as the ftump end. Id. ibid. p. 85, fcq.
The imbibing force of a great variety of trees being tried in the fame manner, by immerfing branches of them in aqueo- mercuiial gages, it was found that the pear, quince, cherry, walnut, pi;ach, apricot, plum, black-thorn, white-thorn, goofeberry, water-elder, and fycamore, raifed the mercury from three to fix inches high. The elm, oak, horfe-chefhut, filbert, fig, mulberry, willow, fallow, ofier, afh, lynden, and curran raifed the mercury only one or two inches. But the ever-greens, and the following trees and plants, viz. laurel, rofemary, lauruftine, phyllerea, fuz, rue, barberry, jtflamine, cucumbci -branch, pumkin, jerufalcm artichoak, &c. did not raife it at all. Id. ibid. p. 101. In order to difcover with what force the fap of the vine is pufhed forth in the bleeding feafon, the following experiments were made. Dr. Hal. s cut off a vine on a weftern afpec"t, within (even inches of the ground ; the remaining ftump hav- ing no lateral branches, and being four or five years old, and i inch diameter. To the top of this flump he fixed a glafs tube, twenty five feet high ; fecuring the joint with ftiff ce- ment made of bees-wax and tuipemine, befides feveral folds of wet bladder faflened over it with Packthread. As the ftem did not atfiril bleed into the tube, he filled the tube two fe:t high with water, which the fame afternoon was imbibed by the ftem within three inches of the bottom. Next day the ftem began to bleed, fo that the fap in the tube continued rifing daily, till itwas above twenty-one feet high, and would very probably have rifen higher, if the joint had not feveral times leaked. In the chief bleeding feafon, it would continue rifing night and day ; but much more in the day than night ; and moil of all in the greateft heat of the day. When the fun fhone hot upon the vine, there was always a continued feries of air bubbles afcending from the ftem through the fap in the tube, in fo great plenty, as to make a large froth on the upper part of the fap, which fhews the great quantity of air drawn in through the roots and item. Id. ibid. p. 108. This force of the rifing fap is five times greater than the force of the blood in the great crural artery of a horfe ; feven times greater than the force of the blood in the like artery of a dug; and eight times greater than the blood's force in the fame artery of a fallow doe. Id. ibid. p. 114. The free communication of the fap vefiels of Plants appears from hence ; that tho' deep gaps or notches be cut in a branch, it will nevcrthclefs imbibe water in the manner already men- tioned. In order to try whether it would not be the fame in branches, as they grow on trees, the Dr. cut two fuch oppo- site gaps in a duck-cherry-branch, three inches diflant from each other, notwithftanding which, the leaves of this branch con- tinued green, within eight or ten days as long as the leaves on the other branches of die fame tree. Id. ibid. p. 129. However, it is the opinion of this excellent author, that there is no uniform circulation of the fap in trees, like that of the blood in animals ; for this reafon, that if there were fuch a circulation, the leaves of the ilex grafted upon the Englifb. oak would fall in winter equally with thofe of the oak itfelf, which is not found to be the cafe. Id. ibid. p. 149. And, that the fap does not defcend between the bark and the wood, as the favourers of a circulation fuppofe, feems evident from hence, viz. that if the bark be taken off for three or four inches breadth quite round, the bleeding of the tree a- bove that bared place will much abate, which ought to have the contrary effect, by interrupting the courfe of the refluent fap, if the fap defcended by the bark. But the reafon of the abatement of the bleeding in this cafe may be accounted for, from hence, that the fap is ftrongly attracted upwards by the vigorous operation of the perfpiring leaves. Id. ibid.
P* I 5°-
We have a great many other curious experiments and obfer- vations, in the fame book, relative to the vegetation and ana- lyfis of Plants ; which are compofed of the following prin- ciples, viz. fulphur, volatile fait, water, earth, and air. See the article Air, Suppl.
The leaves of Plants ferve not only as excretory du£ls to
feparate and carry off the redundant watery fluid, which by
2 being
