424 STEPHENSON'S PATENT 



on each side of the smoke-box to the cylinders, as shown in Plate XCII. Fig. 4 ; they 

 are turned on one side in order to keep them clear of the tubes, so as to allow access 

 to all the tubes, that they may be taken out when necessary through the smoke- 

 box door. It is also necessary to protect the steam pipes from the immediate action 

 of the hot air issuing from the tubes, which is nearly hot enough to melt copper. In 

 the first engines, the steam pipes were brought straight down to the cylinders across 

 the ends of the tubes, but they were found to be very rapidly destroyed. 



The area of the large steam pipe is 19'6 square inches, and is equal to the areas of 

 both the small ones, which are 9'6 square inches each. 



The other end of the steam pipe is connected to the box a, (Plate XC.,) by means 

 of the stuffing box b', containing packing made of loose spun hemp, called gasket, com- 

 pressed against the steam pipe by the brass gland c, which is bolted to a flanch on 

 the stuffing box b'. This stuffing box is necessary because iron expands by heat only 

 two thirds as much as copper, so that when the boiler and its contents are heated, 

 the wrought iron increases in length less than the copper pipe, and the pipe would 

 be very much strained if it were rigidly fixed at both ends, and the joint would soon 

 become defective by the repetition of the action ; but the stuffing-box allows the end 

 of the pipe to slide through it, and still keeps the joint steam-tight, preventing all 

 injurious action*. 



The steam enters through the funnel-shaped copper pipe d', (Plate XC.,) which is 

 fixed upon the top of the box a, and this pipe rises nearly to the top of the steam dome 

 T, which is made of brass, cast three eighths of an inch thick, and is 1 5 inches in 

 diameter and 2 feet high, and bolted down by a flanch on to the fire-box. The 

 object of this steam dome, and of carrying the steam pipe up to the top of it, is to 

 obtain the steam as pure and dry as possible, by taking it at a distance from the 

 water ; because from the great agitation of the water in the boiler, and the rapid 

 emission of the steam to the cylinders, some of the water gets mixed up with the steam 

 in a finely divided state, and is liable to pass over with the steam into the cylinders. 

 This effect is called priming, and is very injurious when it takes place to any extent, for 

 all the water carried over into the cylinder is wasted and occupies the place of steam, and 

 thus diminishes the power of the engine ; but principally because it accumulates in the 

 cylinder and sometimes remains in it, being unable to escape with the waste steam ; 

 and in that case, from being so incompressible, it causes the breaking of some part 

 of the engine. By carrying the steam pipe up into the dome, the quantity of 

 water taken with the steam is very much diminished, as it has time to separate, and the 

 expanded end of the steam pipe nearly filling the dome, serves to catch the water and 

 prevent its entering the steam pipe. 



* The brass tubes of the boiler are liable to the same action, but as they are small and very firmly 

 fixed at the ends, the expansion is immaterial, the tubes allowing for it by bending slightly. 



