428 Progress of Mechanical Science. [Sept. 



prevents tbe carriage running down hill too rapidly, and there is no occasion 

 for drags. 



Mr. J. Farey, engineer, thinks the carriage for conveyance should be upon 

 the same wheels with the engine, to give firmer adherence to the road; approves 

 of the ejection of the waste steam in a stream upwards through a contracted ori- 

 fice at the bottom of the chimney (introduced by Mr. Stephenson on the rail-way 

 engines) , as it increases the draught, which from the necessity of a short flue 

 cannot be maintained otherwise, without fans, blowers, or bellows. However 

 Mr. Summers and other engineers object to this plan, as the contracted orifice 

 of the steam escape takes away proportionally from its power on the other side 

 of the piston. 



The vertical jet gives such an intensity of draught as was never procured before, 

 and with the further advantage, that the rapidity of draught so produced increases 

 whenever the engines work faster and discharge more steam. This may be consi- 

 dered a very important improvement, as is another described by Mr. R. Trevi- 

 thick regarding the construction of boilers. This engineer has taken out a 

 patent for an entirely new engine, wherein the fire-place, boiler, and condenser 

 stand perpendicularly one within the other : they are formed of six wrought iron 

 tubes. One charge of distilled water only is required ; the steam being condens- 

 ed and returned into the boiler by a force pump. To supply the Avaste by leakage 

 a small apparatus is used, which effectually prevents any fluctuation in the height 

 of water in the boiler, or the collection of sediment, and consequeut danger of 

 the boiler becoming heated red-hot. 



The boiler is not only less than any other but stronger, and if it were worked 

 at the same pressure as the portable gas-holders, theory would give a saving of 

 fuel, weight, and room over low pressure engines of sixteen to one. 



Mr. Davies Gilbert explains the apparent anomaly, that with steam power 

 increase of velocity does not enhance expence*. 



It was last year determined by the society of civil engineers, that the ex- 

 pence of conveying carriages drawn by horses was at its minimum, when the 

 rate of travelling equalled about three miles an hour ; and that expence 

 increased up to the practical limit of speed nearly as the velocity; while on 

 the contrary, friction being a given quantity, as well as the force necessary 

 for impelling a given weight up a given ascent, the power required for moving 

 steam carriages on a rail-way remains theoretically independent of its speed, 

 and practically increases a very little in consequence of resistances from the atmo- 

 sphere, slight impacts against the wheels, inertia of the reciprocating piston, &c. 



The expenditure of what may be called efficiency is as the actual force multi- 

 plied by the velocity, and the consumption of fuel in a given time will be 

 in the same proportion : but the time of performing a given distance being 

 inversely as the velocity, the expenditure of fuel in a given time will be constant 

 for a given distance; and it is very nearly so in practice. 



* This is the case on land, where friction is constant, but on water the very reverse 

 takes place ; for the resistance to motion in fluids increases much more rapidly than the 

 velocity : thus the same expenditure of steam (or coal) that will carry a steamer alone 

 a certain distance, with a given velocity, will with a sacrifice of only r l 2 of time, convey 

 her with a large ship astern to the same distance. Vide Gleanings, III. 158. 



