MECHANICS AND USEFUL ARTS. 35 



requisite gearing. Engine and mill empty required, according 

 to one set of diagrams, 74.8 horse-power to run them at the 

 working speed ; but, according to another set of diagrams, the 

 frictional resistance of engine and mill is less than 35 horse-power, 

 and all the diagrams were taken within a few hours. We cite 

 this case only to illustrate the difficulties engineers have to contend 

 with in endeavoring to estimate the friction of engines under 

 ordinary circumstances. 



The other experiment is very interesting and curious as regards 

 results. The engine was a double cylinder traction engine, built 

 by Messrs. Howard, of Beckford. The cylinders are 8 inches 1 

 diameter and 12| inches' stroke. The engine shaft can be discon- 

 nected from all the rest of the machinery, so that the whole work 

 done by the steam consists in turning the crank shaft and over- 

 coming the friction of the bearings, pistons, etc. With 60 pounds 

 of steam in the boiler, the engine, making 190 revolutions, indi- 

 cated unloaded 2.64 horse-power. The engine Avas then set to 

 drive a brake loaded to 16 horse-power, the link being put in full 

 gear; under these conditions the engine indicated 22.55 horse- 

 poAver. The frictional resistance Avas therefore increased, by the 

 fact that the engine Avas HOAV doing Avork to 6.55 horse-power, or 

 to nearly 3 times that of the unloaded engine. This is all plain 

 sailing, but noAV comes a most remarkable fact. The throttle 

 valve Avas thrown full open, or nearly so, and the engine linked 

 up, that is, worked expansively at the same velocity, 190 revo- 

 lutions per minute. The load on the brake, etc., remaining abso- 

 lutely unaltered, any engineer Avould predict that, under these 

 circumstances, the result Avould be the same. Far from this being 

 the case, hoAvever, it Avas noAV found that, the effective Avork or 

 duty of the engine being unaltered, the indicated power was only 

 19.86 horse-power, so that the friction of the engine Avhen linked 

 up Avas only 3.86 horse-power, or little more than one-half that of 

 the engine Nvorking in full gear. Lest there should be any mis- 

 take about this, the brake Avas then loaded with 504 pounds. 

 With the link in full gear, the engine indicated 44.88 horse-poAver ; 

 the link Avas then put in the first notch, and the throttle valve 

 fully opened, everything else remaining unchanged, when the 

 power fell to 40.92 horse ; the frictional or internal resistance of 

 the engine in the latter case thus being 3.86 horse-powerless than 

 in the immediately preceding experiment. HOAV are these facts to 

 be accounted for? Is it that the varying strain on moving 

 surfaces in contact, due to the action of expanding steam, is 

 attended Avith less frictional resistance than is present when the 

 metals are under the steadier strain of non-expanding steam? 

 We shall not pretend to answer these questions. There are the 

 facts for the consideration of those interested. 



Is it too much to hope that engineers, Avho have the opportu- 

 nity, Avill take up this subject and endeavor to throw light into 

 what is at present a A~ery dark and unexplored region of mechan- 

 ical engineering? We are convinced that the results Avould, 

 Avhen time and perseverance had multiplied data, be found of 

 very great value to those Avho desire to see the steam engine un- 



