Chap. 5.] formidable Effe&s of Steam. 127 



his Phyfiological Difquificions, ferves well to mark its 

 effects. * A workman, who with fome others was em- 

 ployed to repair a fleam -engine at Chelfta, informed 

 me, that as they were bufy about it in working it to 

 imderftand the defect, the barrel, which was of great 

 capacity, and too much worn with long ufe, burft on 

 a hidden, and a cloud of fteam rufhing out at the frac- 

 ture (truck one of the workmen who was (landing by, 

 and killed him in a moment like a blaft of lightning. 

 His fellows ran up as foon as they could to give him 

 alTiftance, but when they endeavoured to take off his 

 cloaths, the fiefh came away from the bones along with 

 them/ 



Though all fluids are rendered elaftic by fire, yet 

 the quantity of caloric neceffary to raife them to a ftate 

 of vapour depends upon different circumflances. The 

 very nature of an elaftic fluid renders it particularly 

 liable to be affected by the weight which is incumbent 

 upon it. All fluids therefore boil with a much lels 

 degree of heat in vacuo, than under the ordinary prei- 

 fure of the atmofphere. Thus water moderately heat- 

 ed, and placed in the receiver of an air-pump, may 

 be made to boil by withdrawing a part of the air by 

 which it is comprefied ; and it will be obferved to 

 ceafe as foon as the air is returned, and the preffurc 



metal is raifed : when the pifton is to fall, the fteam is fuddenly 

 made to collapfe by the injection of fome cold water, which, im- 

 mediately condenfes it, and makes a vacuum, fo that the pifton is 

 forced down again by the preflure of the atmofphere. By thefe 

 alternate rifings and fallings of the pifton, feveral of which are 

 performed in the fpace of a minute, the machine afls on the work 

 of a forcing pump, by which the water is raifed, and difcharged at 

 the proper place. This machine, confidering the vaft force of it, 

 is one of the fimpleft in the world; but, like the digefter, may be- 

 come extremely dangerous if .-.ny accident fhould get 

 the power over it. 



reftored. 



