ON RAII.-ROADS. 275

improve the economy of this kind of engines,should be directed.

A great saving will be observed in the con-struction of the two engines, No. 1 and No. 2,by increasing the dimensions of the tubethrough the boiler, and thus exposing agreater area of water to the action of the fire.No doubt., a similar increase of saving will befound, by further enlarging the surface of thefire; a circle, however, does not expose thegreatest surface in a given area. I have latelyput an oval tube into one of the engines uponthe Killing-worth Rail-road, but not yet havinghad an opportunity of subjecting it to trial, Icannot at present give the result.

Mr. Watt states, that, in the most judiciousfurnaces, it requires eight feet of surface ofboiler to be exposed to the action of the fireand flame, to boil off’ a cubic foot of water inan hour. In the Tables VI. and VII. we findthe quantity of water used was about 120gallons an hour; six gallons is nearly equalto a cubic foot, which will make the evapora-tion about twenty cubic feet of water in anhour; and, by Mr. Watt’s rule, the extent ofthe surface should be 160 feet; if the tube wastwo feet diameter and nine feet long, the wholesurface would not be more than fifty-four feet,

JS ^