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346 LECTURE XXIX. 



and such a column would represent the pressure causing the draught, which 

 might, therefore, be expected to have a velocity of 6 feet in a second. If 

 the room were perfectly closed, the air contained in it would by degrees 

 become so much lighter than the «xternal air, as would be equivalent to one 

 foot of the height of the column causing the pressure, and the current 

 would then stop; if fresh air were gradually admitted by a small ori- 

 fice, the current would again go on, but the air in the room would always 

 remain somewhat rarer than the external atmosphere, unless a fresh supply 

 were admitted through ample openings. 



The object of a chimney is not so much to ventilate the room, as to pro- 

 vide a sufficiently rapid supply of air for maintaining the process of com- 

 bustion, and to carry off" the products of that process : hence, it is desirable 

 to allow as little air as possible to enter the chimney without passing through 

 the fire ; and this is the best general mode of avoiding smoky chimnies. 

 For wind furnaces, the flue should be as equable as possible, throughout its 

 height, or widened rather than contracted in its ascent, and free from any 

 considerable angles. 



'»' 



The ascent of a balloon is an effect of the same kind as that of air in a 

 chimney, and arises sometimes from the same cause, when the air within it 

 is expanded by heat ; but more commonly from the greater rarity of hydro- 

 gen gas, with which the balloon is filled, and which, when pure, is only one 

 thirteenth as heavy as atmospherical air, but as it is commonly used, about 

 one fifth or one sixth. 



The steam engine is perhaps the most magnificent effort of mechanical 

 power; it has undergone successive changes, and it appears to have been 

 brought very near to perfection by the improvements of Mr. Watt. The 

 pressure of steam Avas first applied by the Marquis of Worcester, and after- 

 wards by Savery, to act immediately on the surface of water contained in a 

 close vessel, and this water was forced, by the elasticity of the steam, to ascend 

 through a pipe. But a great degree of heat was required for raising water to 

 any considerable height by this machine : for, in order that steam may be 

 tnade capable of supporting, in addition to the atmospherical pressure, a 

 column of 34 feet of water, its temperature must be raised to 248" of Fahieu'- 



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