A x-reRsel, in hydraulics, is a name given to those metalline cylinders, which are placed between the two forcing pumps in the improved fire-engines. The water is injected by- the action of the pistons through two pipes, with valves, into this vessel; the air previously con tained in it will be compressed by the water, in proportion to the quantity ad mitta and by its spring force the water into a pipe, which will discharge a con stant and equal stream; whereas, in the COMM= squirting engine, the stream is discontinued between the several strokes. Other water-engines are furnished with vessels of this kind.
Ant-reseels, in botany, are certain ca nals or ducts, whereby a kind of absorp tion and respiration is effected in vegeta ble bodies.
Air-vessels have been distinguished from sap-vessels ; the former being sup posed to correspond. to the trachea and lung,s of animals ; the latter to their lac teals and blood-vessels.
Dr. Grew, in an inquiry into the motion and cause of the air in vegetables, shews, that it enters them various ways, not only by the trunk, leaves, and other parts above ground, but at the root. For the reception, as well as expulsion of the air, the pores are so very large in the trunks of some plants, as in the better sort of thick walking-canes, that they are sisible to a good eye without a Flass ; but with a glass, the cane seems as if it were stuck full of large pin-holes, resembling the pores of the skin in the ends of the fin gers, and ball of the hand. In the leaves of the pine, through a glass, they make an elegant shew, standing almost exactly in nu& and file throughout the length of' the leaves. But though the air enters in
partly at the trunk, and also at other parts, especially in some plants, yet its chief admission is at the root: much as, in animals, some part of the air may con tinually pass into the body and blood by the pores of the skin ; but the chief draught is at the mouth. If the chief en. trance of tbe air were at the trunk, before it could be mixed with the sap in the root, it must descend; and so move not only eon tray to its own nature, but in a contrary course to the ap : whereas, by its recep tion at the root, and its transition from thence, it has a more natural and easy motion of ascent. The same fact is far ther deduced from the fineness and small ness of the diametral apertures in the trunk, in comparison of those in the root, which nature has plainly designed for the separation of the air from the sap, after they arc both together received into them.
Air-vessels are found in the leaves of all plants, and are even discoverable in many without tbe help of glasses ; for, upon breaking the stalk or chief fibres of a leaf, the likeness of a fine woolly sub stance, or rather -of curious small cob webs, may be seen to hang at both the broken ends. This is taken notice of, not only in some few plants, as in scabious, where it is most visible : but may also be seen more or less in most others, if the leaves be very tenderly broken. This wool is really a skein of air-vessels, or rather of the fibres of the air-vessels, loosed from their spiral position, and so drawn out in length.