254 MOTIVE POWER EMPLOYED

friction of the carriages. The latter I havepreviously shewn ; 1 shall, therefore, give anexperiment made to ascertain the former.

EXPERIMENT XXXIV.

Upon the same plane or piece of Rail-roaddescribed in Experiment V., with a descent of11 feet 2 inches in 388 yards, I allowed 5 loadedcarriages, each weighing 9408 lbs., to descendfreely, as explained in that experiment, whichthey performed in 120 seconds. I then hadthem drawn up the plane again, and attachedthe engine to them, shutting off the communi-cation between the boiler and the cylinders ;so that the steam could not act upon thepistons, either to accelerate or retard the mo-tion of the engine; the top and bottom of thepiston being alternately open to the atmos-phere ; the only obstruction to the motion ofthe engine down the plane was then the fric-tion of the various moving parts. Tho car-riages and engine was allowed to descend theplane freely, which occupied 150 seconds,making the obstruction of the engine, to thegravitating force of the waggons, equal to 30seconds.

The weight of the engine and convoy-carriages together was9 tons := 20160 lbs., and the carriages 9408 X 5 — 47040lbs.,making together 20160 + 47040 = 67200 lbs.