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 atmosphere, 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.
