412

Observations on the device of Decaus.

[Book IV.

one knows that when the covers of these fit so close as to prevent thesteam from escaping as fast as it is generated, the confined vapor forces upthe hot liquid through the spouts; and in a manner precisely the same asdescribed by Decaus, for the effect is the same whetlier the dischargingtube be connected to the lower side of a boiler like a tea-kettle spout, orinserted through the top and continued within to the liquid. From suchdomestic exhibitions of the effects of steam, the devices of Heron andother ancient experimenters were probably derived : a person whosethoughts were turned to the subject of raising water by it could not fail toprofit by them, or to hit upon so slight a modification of the apparatus asshown in the last figure.

The same application of steam was often exhibited by alchymists as al-ready observed in their manipulations, and in drawing off the contents oftheir stills and retorts; but it was still more clearly illustrated in commonlife in the employment of eolipiles, and the copper ball of Decaus wasmerely one of these with ajetpipe prolonged intothe liquid. The very terms“ball of copper,” “ball of brass,” were those by which eolipiles weredesignated. (See page 396.) Now no one was ignorant that an openingon the top of one of these instruments let out steam, and that through onenear the bottom hot water would be violently expelled through a verticaltube, if attached to the opening. Suppose the one figured at No. 185either accidentally or designedly placed on the fire with the tube inclinedupwards, and heated in that position while two thirds filled with water;the vapor would then accumulate in the dome, and would necessarilydrive out the boiling liquid until the lower orifice of the tube was no longercovered with water: or imagine No. 184 inclined tili water rushed outinstead of steam. That such experiments were not only frequent but com-mon, no person can reasonably doubt, although no notice of them may befound in books. Such a mode of raising water was off little value and notthought worth recording, and but for its introduction into some historiesof the steam-engine, we should not have deemed it of sufficient importanceto notice. Moreover, the ordinary mode of charging eolipiles which hadbut one minute orifice, viz. by lieating and then plunging them in water,must have frequently caused them to produce liquid jets, in consequenceof their imbibing too much, and there being no other way of expellingthe surplus than by placing the instrument on the fire. Probably an eoli-pile was never used that was not occasionally overcharged with the liquid,and thus niade to raise a portion of it by the elastic force of steam. Atany rate, no one who was familiär with these instruments, from Heron toDecaus, could have been ignorant of the fact that they might be applied toproduce jets of hot water as well as of vapor; and few ever used themwho did not occasionally make them produce both.

It would be an unjust reflection on Decaus to suppose he could not havegiven a better plan than No. 188 for raising water by steam, if the projecthad been seriously ent.ertained by him; but there is not the slightest groundto believe he ever dreamt of applying that fluid to hydraulic purposes, oras a Substitute for pumps, chains of pots, &c. He certainly would havelaughed at any one proposing a device by which water could not be raiseduntil the whole of it was boiled, whether the quantity was a pint, ahogs-head, or a million of gallons; and in some cases not until its temperaturefar exceeded that at which ebullition in open vessels takes place. Whythen, it may be asked, did he mention the device at all] Simply to showthat “ water may be raised above its level by means of fire.” Well, buthe says that “diverse machines” may be deduced from it. True, and hehas given a description of one, from which we may judge of the rest: these