26.  This reasoning ought, however, to be admitted with caution; and perhaps some inducement to examine it carefully may be presented by tracing it to extreme cases.  It would seem, but this is not a necessary consequence, that a gun might be made so long, that it would burst although no obstacle filled up its muzzle.  It should also follow that if, after the gun is charged, the air were extracted from the barrel, though the muzzle be then left closed, the gun ought not to burst.  It would also seem to follow from the principle of the explanation, that a body might be projected in air, or other elastic resisting medium, with such force that, after advancing a very short space it should return in the same direction in which it was projected.

Notes: 

1.  See Poisson’s remarks, Ecole Polytec.  Cahier, xxi, p. 191.

Chapter 3

Regulating Power

27.  Uniformity and steadiness in the rate at which machinery works, are essential both for its effect and its duration.  The first illustration which presents itself is that beautiful contrivance, the governor of the steam-engine, which must immediately occur to all who are familiar with that admirable engine.  Wherever the increased speed of the engine would lead to injurious or dangerous consequences, this is applied; and it is equally the regulator of the water-wheel which drives a spinning-jenny, or of the windmills which drain our fens.  In the dockyard at Chatham, the descending motion of a large platform, on which timber is raised, is regulated by a governor; but as the weight is very considerable, the velocity of this governor is still further checked by causing its motion to take place in water.

28.  Another very beautiful contrivance for regulating the number of strokes made by a steam-engine, is used in Cornwall:  it is called the cataract, and depends on the time required to fill a vessel plunged in water, the opening of the valve through which the fluid is admitted being adjustable at the will of the engine-man.

29.  The regularity of the supply of fuel to the fire under the boilers of steam-engines is another mode of contributing to the uniformity of their rate, and also economizes the consumption of coal.  Several patents have been taken out for methods of regulating this supply:  the general principle being to make the engine supply the fire with small quantities of fuel at regular intervals by means of a hopper, and to make it diminish this supply when the engine works too quickly.  One of the incidental advantages of this plan is, that by throwing on a very small quantity of coal at a time, the smoke is almost entirely consumed.  The dampers of ashpits and chimneys are also, in some cases, connected with machines in order to regulate their speed.