ON PNEUMATIC MACHINES. 269 



of an air gun, an instrument of considerable antiquity, but of little utility. 

 It- is obvious that no human force can so far increase the density of 

 air as to make its elasticity at all comparable to that of the fluid evolved 

 by fired gunpowder, and even if it were reduced to such a state, its effects 

 would still be far inferior to those of gunpowder : for the utmost velocity, 

 with which it could expand itself, would not exceed 1300 feet in a second, 

 and it would, therefore, be incapable of imparting to a ball a velocity even 

 as great as this, while the vapour of gunpowder impels a heavy ball with 

 a velocity of more than 2000 feet in a second. When, however, it is 

 considered that by far the greatest part of such a velocity as this is use- 

 lessly employed, and that the mechanical power which is practically 

 obtained from gunpowder is much more expensive than an equivalent 

 exertion of any of the ordinary sources of motion, it must be allowed 

 that the force of condensed air may possibly be applied in some cases with 

 advantage, as a substitute for that of gunpowder. (Plate XXIV. Fig. 339.) 



[The improvements which have been effected in the construction of con- 

 densing steam engines since the time of the publication of these Lectures, 

 are neither few nor unimportant. As, however, most of them are con- 

 nected with details, rather than with principles, it will not be necessary to 

 give a very specific account of them. They consist of alterations in the 

 construction of furnaces and the regulation of the fire ; better forms of the 

 boiler and its appendages ; simpler modes of effecting a communication 

 between the boiler, the cylinder, and the condenser, by a new form of the 

 valves, and an improved way of opening them ; and, lastly, more accurate 

 methods of fitting the different portions together, so as to lose less heat and 

 to waste less steam. Many of these improvements are the results of prac- 

 tical experience. About 1811, the proprietors of some of the Cornish mines 

 established a system of inspection of their engines, the efficacy of which is 

 fully evinced by the work of the registrar and inspector, Captain Lean. 

 He mentions the following instance of it,* " relative to Stray Park engine, 

 a single engine, on Boulton and Watt's construction, of sixty-inch cylinder. 

 When this engine was first put on the report in 1811, its duty was below 

 1C millions : during eight months, ending with April 1813, it had consumed 

 17,633 bushels of coal, performing the average duty of 21 '5 millions, and 

 worked at the rate of 5 strokes per minute : during eight months, ending 

 with April 1814, it had consumed only 12,671 bushels of coal, performed 

 the average duty of 30'5 millions, and worked at the rate of 5'7 strokes per 

 minute." And from the same work it appears, that the average duty had, 

 up to 1834, increased from 26'5 to 90 millions ; the duty being the number 

 of pounds which are raised one foot high by a bushel of coals. The eco- 

 nomy of the Cornish boiler and its appendages is due, in a great measure, 

 to the extent of surface which is presented to the flame. This is effected 

 by a number of flues, external and internal, the latter somewhat analogous 

 to those of the locomotive boiler, which will be described presently. The 

 firg is laid on in large masses, and allowed to consume slowly, whilst the] 



* Historical Statements of the Improvements on the Duty of Engines in Corn- 

 wall. By T. Lean & Brother, 1836. Introd. p. 11. 



