500

Raising Water with Open Tubes.

[Book V.

be sunk perpendicularly in water, except one or two inches by which it isheld, and then raised eight or ten inches, air will enter the small orificeand fill the part previously occupied by the liquid : if the upward move-ment be very slow, the air will gradually fill the interior without disturbingthe surface of the liquid; but if the tube be raised by a rapid movementor slight jerk, the air will then rush into the void with a force that willpush down the liquid before it to a considerable depth, so that%n the re-ascent of the liquid its momentum will project a portion in the form of ajet, precisely like Nos. 224, 225 and 226. It is surprising how elevateda wave is generated in the tube by the slightest ascent of the latter, pro-vided its movement be made sufficiently quick. The rise of the water,too, follows that of the tube so rapidly that most observers at first supposethem to rise simultaneously. The fact is, the liquid when depressed re-turns with such velocity as to escape from the tube the instant the strokeis finished, and even before its motion be slackened.

Exper. VI. A jet may be produced by the descent of the tube as well asby its ascent. Let No. 228 be so held that its lower end dip not.more thanan inch or an inch and a half in the water, and then be pushed quicklydown eight or ten inches—a stream will be projected from its upper ori-fice to an elevation of six or seven feet, and will be instantly followed byanother that will reach nearly as high. The same cause operates here asin the upward movement, but it is differently excited. A small part onlyof the air within is expelled at the end of the stroke, on account of thetube’s rapid descent, and consequently the water is prevented from enter-ing ; but as soon as this movement of the tube ceases, the liquid rushes inand a portion ascends in the form of a jet. On the subsequent ebb of thewave within, another one rises nearly equal to the first, and causes thesecond jet. The following experiment will illustrate both movements :—A small glass tube eight inches long, its wide end an inch and five-eighthsdiameter and its small end one-eighth, was employed. By its upwardmovement or stroke the extremity of the jet reached to an elevation of ninefeet. By the downward stroke a jet rose six feet, which was succeededby another that reached four feet and a half. Now if both movementsare properly combined in a spouting tube of large dimensions, we believethe instrument may be made to raise as much water, in circumstancesadapted to its employment, as any other hydro-pneumatic machine.

If the figure given to No. 226 should be found better adapted than anyother when the tube is used as a siphon, it does not therefore follow thatthe same form would be the most suitable to produce jets of water. Inthe former case the instrument acts while at rest, but in the latter a con-stant and rapid movement is required : hence, to prevent an unnecessaryexpenditure of the power employed, it should be so formed as to presentas little opposing surface to the resistance of the dense fluid in which itworks as is consistent with the elevation, or quantity of water to be raisedby it. This remark applies particularly to the lower or wide end, for ifthat part be suddefily expanded or flared like a trumpet, a volume of waterof equal diameter has to be displaced in the reservoir every time the tubeis pushed down, and also a ring of water whose external diameter is thesame (the internal one being bounded by the tube) every time the latteris lifted up. When used as spouting tubes the lower end should there-fore flare very little, if any, unless in cases where the outlay of power towork them is of little consequence or of secondary importance. Theupper end of a spouting tube, when intended to throw jets from its orifice,should not diverge like that of No. 226, since the elevation of the streamwould be thereby diminished : instead of rising in a compact jet, it would