360

Herrn’s Fountain.

[Book III.

water by it. How many ages have elapsed, and how many millions ofpeople have witnessed this Operation, without a useful idea having beenderived from it I And without any one thinking that valuable lessons inScience might be learnt from a disordered pump, or from the irregulärmovements of a pump handle 1 Those observing minds, however, thatare constantly on the alert for facts—like bees incessantly on the wing forhoney—would not now suffer even such an occurrence to pass unnoticed;nor would they hesitate to consider those unpleasant knocks whichhundreds of people (and the writer among them) have occasionally expe-rienced from the unexpected descent of a heavy pump handle on theirpersons, and in some instances more unpleasant ones from its sudden as-cent—as admonitions to turn the experiment to advantage. The simplerise of water which his body displaced in a bathing tub, was seized in atwinkling by the mathematician of Syracuse to solve a new and difficultproblem ; yet the same thing had been previously witnessed for thousandsof years, but no one ever thought of applying the result to any suchpurpose.

It perhaps may be a question whether the machines already describedin this chapter were known to the engineers of antiquity, but there is noroom to doubt their acquaintance with another variety of pressure engines,since we have obtained a knowledge of them from the Spiritalia of Heron,whose name they still bear. It is obvious that a liquid may be forced outof a vessel by pressing into the latter any other substance, no matter whatthe nature of it may be, whether solid or fluid, liquid or aeriform: thus,the solid plunger or piston of a pump does not more effectually expel theContents of the cylinder in which it moves, than the elastic fluid in a sodafountain drives out the aerated water; bence, if air be urged by the pres-sure of a liquid column, or by any other force, to occupy the interior of avessel containing water, the liquid may be raised through a tube to anelevation equal to the force that moves it; the air in this case performingthe part of pistons in the pressure engines already described ; and its ef-fects are greater than can be produced by solid pistons, for the friction ofthese consumes a considerable portion of the motive force, so that a co-lumn of water raised by them can never equal the one that raises it;whereas air, from its extreme mobility, receives and transmits the momen:tum of the motive column undiminished to the other.

The fountain of Heron is the oldest pressure engine known, and in it avolume of air is used as a Substitute for a piston. It is not certain that itwas invented by him, for it may have been an old device in his time,and one which he thought worthy of preservation, or of being made moreextensively known, and therefore inserted an account of it in his book.See No. 163. The two vessels A B, of any shape, are made air tight.The top of the upper one is formed into a dish or basin; in the centre ofwhich the jet pipe is inserted, its lower end extending to near the bottomof A: a pipe C, whose upper orifice is soldered to the basin extendsdown to near the bottom of the lower vessel, either passing through thetop of B, as in the figure, or inserted at the side. Another pipe D is con-nected to the top of B, and continued to the upper part of A. This pipeconducts the air from B to A. Now suppose the vessel A filled withwater, through an aperture made for the purpose, and which is thenclosed ; the object is to make this water ascend through the jet, and it isaccomplished thus:—water is poured into the basin, and of course it runsdown the pipe C into B ; and as it rises in the latter, the air within is ne-cessarily compressed, and having no way to escape but up the pipe D, itascends into the upper part of A, where being pressed on the surface of