1168

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Fig. 1770,

It has been proposed to employ a fall of water to turn the Archimedean screw, by addingto the extremity of the barrel a second screw, having the barrel as its newel, and itsfillets inclined in a direction contrary to those of the first. This idea was suggested by theMarquis of Worcester in his “ Century of Inventions.”

Calculation of the Effects of the Archimedean Screw . — The first of the two following ex-periments was made by Lamande. The screw was 5*85 metres long, and 049 metresdiameter ; it was turned by two relays of men, working alternately two hours in succession :each relay consisted of 9 men ; the winch made 40 turns per minute. The quantity ofwater elevated in an hour was 45 cube metres, and the height was 3-3 metres: the 18 menin two relays elevated in 10 hours’ daily labour 450 cube metres of water 3*3 metres, or1485 cube metres 1 metre, which gives for the measure of a man’s daily work 82 units of acube metre of water elevated 1 metre, a power nearly equal to that of a man employed indriving piles.

The following experiment gives 10 units more per man for the useful effect of the screw :six men worked six hours a day; the screw made 35 turns per minute; they elevated in oneminute 765 litres to 2 metres, or 91*8 cube metres of water to 1 metre.

The motion given to the water beyond the screw consumed a great part of the movingpower ineffectually, and the less the screw plunges into the water, the more its usefulelfect will increase with relation to the power employed in turning it. The external motionis one cause of the loss of power, which increases with the dimensions of the screw. Thesame observation has been made with regard to screws for blowing. These machines haveexternally the form of a short drum, whose diameter is very great in comparison with theordinary barrel of the screw; the part plunged in a reservoir of water impresses on it amotion which has no effect in relation to the object required.

Suction Pump. — The simple suction pump consists of two pipes A B, C D.(fig. 1771.) thediameter of the second being much greater than that of the first, united by two flanges E, F,cast with the pipes, pierced with four holes to admit the screws C, C, which are fitted withnuts, to screw the flanges together, between which the two roundles of leather are placed.The pipe A B, which dips in the water to be raised, V Z, is called the suction pipe; its lowerextremity is slightly widened, to admit the water more freely ; at A A is a metal platepierced with several holes that the water may not carry any dirt with it. The pipe, C I),generally of copper or brass, is called the barrel; it is well polished inside to enable the pistonto work easily, it being desirable that the friction should be diminished as much as possible.

The piston is an inverted truncated cone, OPLK, the base of which is surrounded by aleather band fixed by one or two rows of tacks driven close together ; this band should beslightly funnel-shaped at the upper part, and fit the pump-barrel tightly where the pistonis introduced, the diameter of which should be two-twelfths of an inch less. These kindsof pistons are generally of hornbeam or alder, being less subject to split than other wood ;iron rings are fitted to their two bases to give great durability. The piston has a holethrough its centre, M, closed with a valve, N, of leather, attached to the wood by a tailserving as a hinge; the valve when closed should exceed by half an inch the circumferenceof the hole, and to fit more exactly, it is loaded with a plate of lead ; it has also a fork O Q P,to which an iron rod, R, is attached.

In the middle, E F, of the junction of the pump-barrel with the suction pipe is another