65 



sibility of exceeding in a small compass the power of the Glass 

 and Porcelain furnaces, and to a certain degree he succeeded, 

 but his plan was defective as the area of his flue was only tV of 

 the area of the furnace, he also entertained an erroneous idea that 

 the draught was augmented by a horizontal tube connected with 

 the bottom of the furnace. Macquer's furnace was very good, 

 his attempt to explain the theory of the drauglit is, however, 

 defective. He supposes that the air in the upper part of the 

 furnace is rarified so as to form a vacuum into which the ex- 

 ternal air presses ; he, however, does not explain why the air 

 ascends rather than descends ; but Baume has supplied this de- 

 ficiency by saying, that fire, being light, by its own nature must 

 ascend. Macquer proceeds to state, that the current of air is 

 more strong, as the vacuous space at the top of the furnace is 

 the greater, and therefore it is useful to enlarge this space by 

 the addition of a tube (tuyau d'aspiration) ; and to give the cur- 

 rent velocity we must make its aperture less than that of the 

 furnace, for fluids urged through a contracted channel must 

 move more rapidly as the contraction is greater, (i) 



This explanation may be a lesson to those who attempt to 

 account for any phaenomenon without the necessary knowledge ; 

 for Macquer, notwithstanding his excellence as a chemist, must 

 have been completely ignorant of Hydrostatics, and ought to 

 have consulted some of his mathematical friends on the subject. 

 He, however, found that his principles were faulty ; for if the 

 tube was too much contracted, the performance of the furnace 

 was materially affected, and he supposed this to depend on tlie 



VOL. XIII. L 



{b) See his Diet, de Chj'mie. Fourneau. 



