PUMP, in hydraulics, a machine form ed on the model of a syringe, fur raising of water. See. HYDRAULICS. PUMP air. See Pump chain, consists of a tong chain, equipped with a sufficient number of valves, at proper distances, which working upon two wheels, one above and the other below, passes downward through a wood en tube, and returns upward through another. It is managed by a long winch or roller, whereon several men may be em ployed at once, and thus it discharges, in a limited time, a much greater quantity of water than the common pump, and with less fatigue, and inconvenience to the labourers. This machine was formerly exposed to several disagreeable accidents, by nature of its then construction. The chain was of too complicated a fabric, and the sprokel wheels, employed to wind it up from the ship's bottom, were defi cient in a very material circumstance, viz. some contrivance to prevent the chain from sliding or jerking back upon the surface of the wheel, which frequently happened when the valves were charged with a considerable weight of water, or when the pump was violently worked. The links were evidently too short, and the unmechanical manner in which they were connected exposed them to a great fric tion in passing round the wheels. Hence they were sometimes apt to break or burst asunder in very dangerous situa tions, when it was extremely difficult, and sometimes impracticable to repair the chain. Of late, however, some consi
derable improvements have been made by Mr. Cole, under the direction of Captain Bentinck. The chain of this machine is more simple and mechanical, and less ex posed to danger. It appears to have been first applied to the pump by Mr. Mylne, to exhaust the water from the caissons at Blackfriar's Bridge. It was thence transferred to the marine by Cap, tain Bentinck, after having received some material additions to answer that service. The principal superiority of this pump to the former is, 1. That the chain is more simple and easily worked, and conse quently less exposed to injuries by fric tion. 2. That the chain is secured upon the wheel, and thereby prevented from jerking back when charged with a co lumn of water. 3. That it may be easily taken up and repaired when broken or choked with ballast, &c. And, 4 That it discharges a much greater quantity of water with an inferior number of men. This has been proved by experience, when two men (instead of four) discharg ed a tun of water in fifty-five seconds.