154 MOTIVE POWER AND DISPOSITION

in all the different states of the weather ; Ishall afterwards give an account of some ex-periments made to ascertain this point.

From the nature of this kind of motivepower, it will occur to the reader, that theascent in the line of road must never be suchas will present a resistance to the engine, equalin amount to the adhesion of its wheels uponthe rail; the limits of their action will, there-fore, be within certain lines of road, nearlyapproaching to a level, or receding little fromit. When the amount of adhesion shall havebeen ascertained, we will be able to fix thislimit.

Since writing the foregoing, I have beenfavored w ith a drawing of one of the engines,at present used upon the Wylam collieryRail-road, previously noticed ; which, as itsconstruction is different from the Killingsworthengines, and adapted for a plate Rail-way, andbeing, also, supported by eight wheels, think-ing a plan of it might be useful, 1 have madea plate of it on a larger scale.

Plate VI. will shew a side-view of this engine; B repre-sents the boiler, which is circular, with one hemisphericaland one flat end. The tube in this boiler does not passdirectly through and terminate at the chimney, as shewn inthe other engines, but passes from the end a to the end bof the boiler, and then returns back again to the chimney,at the end a. thus forming a double tube in the interior