ON PNEUMATIC MACHINES. 265 



in a second. If the room were perfectly closed, the air contained in it 

 would by degrees become so much lighter than the external air, as would 

 be equivalent to one foot of the height of the column causing the pres- 

 sure, and the current would then stop ; if fresh air were gradually ad- 

 mitted by a small orifice, the current would again go on, but the air in 

 the room would always remain somewhat rarer than the external at- 

 mosphere, 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 was first applied by the Marquis of Worcester,* and 

 afterwards by Savery,f 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 made capable of supporting, in addition to the atmospherical pres- 

 sure, a column of 34 feet of water, its temperature must be raised to 248 

 of Fahrenheit, and for a column of 68 feet, to 271 ; such a pressure, also, 

 acting on the internal surface of the vessels, made it necessary that they 

 should be extremely strong ; and the height to which water could be drawn 

 up from below, when the steam was condensed, was limited to 33 or 34 feet. 

 A still greater objection was, however, the great quantity of steam neces- 

 sarily wasted, on account of its coming into contact with the cold water 

 and the receiver, the surfaces of which required to be heated to its own 

 temperature, before the water could be expelled ; hence a tenth or a twen- 

 tieth part only of the steam produced could be effective ; and there would 

 probably have been a still greater loss, but for the difficulty with which 

 heat is conducted downwards in fluids. These inconveniences were in 



* See p. 278. There is reason to believe that Hooke, in 1678, was master of the 

 principle ; for he gives in a cypher the outline of " a very extraordinary invention in 

 mechanics, above the chimeras of perpetual motion, for several uses." The cypher is 

 expressed by Pondere premit aer vacuum quod ab igne relictum est. Waller's Life 

 of Hooke, p. 21. 



f Ph. Tr. 1699, p. 228, with a plate of the engine. Improvements on it by De 

 Moura, Ph. Tr. 1752, p. 436. 



