24 TECHNOLOGY. 
ported on railways is that of horses, steam, atmospheric pressure, or gravity. 
The employment of horses on railroads differs from that on ordinary roads 
only in their being able to draw much heavier loads. We therefore proceed 
at once to the consideration of the locomotive steam-engines and cars. The 
employment of atmospheric pressure or gravity requiring special modes of 
construction, we shall treat of them under separate heads. 
a. Locomotives. The general features of a locomotive, aside from the 
tender which carries the supply of fuel and water, are the following: A 
tubular boiler is supported on a frame with four, six, or eight wheels wedged 
firmly on their axles, which turn in bearings. Below the boiler or on both 
sides of the frame are two cylinders, the piston-rods of which cause the axles 
of the driving-wheels to revolve either by means of cranks or by wrists on 
the outside of the wheels. The other wheels either revolve independently, 
in which case they are smaller than the drivers, or they are coupled with 
the latter by connecting rods, when they must be of exactly the same 
diameter. 
Pl. 4, fig. 1, is a side view of a locomotive, jig. 2 a vertical section 
through one of the cylinders, jig. 3 a horizontal section in the plane of the 
sliding-valves, and jigs. 4, 5, and 6, represent the apparatus for working the 
valves and reversing the motion in different positions of the eccentrics. a 
is the boiler, c the fire-box, r the smoke-box, and «@ the chimney. The 
boiler is cylindrical, and is made of sheet-iron of about 4 of an inch in 
thickness, riveted steam-tight with 3 inch rivets. It is covered by a 
casing of strips of inch plank, hooped together to diminish the radiation of 
heat. 
The fire-box has double sides, the inner being of sheet copper; it descends 
about two feet below the bottom of the boiler. The grating is in the middle 
of the bottom part. It is seen by jig. 2 that the fire-box is surrounded by 
water in all parts but the door and the grating. The tubes or flues extend 
from the fire-box to the smoke-box, and are entirely surrounded by water ; 
there are from sixty to one hundred and eighty flues in a boiler, and it is 
the large amount of heating surface gained by this arrangement that con- 
stitutes the superiority of the tubular boilers over all others in the production 
of steam. If any of the flues collapse, the water will enter the fire-box and 
put out the fire, but no explosion will ensue. 
Below the smoke-box are the two steam-cylinders vv. Above the fire- 
box is the steam-dome p, into which the steam rises before passing on to the 
cylinders, in order to deposit the particles of water which it carries with it. 
The steam then descends as the arrow shows through a funnel, and passing 
along the pipe s arrives at the cylinders, as shown by the second arrow. 
The enlarged portion of the steam-pipe is screwed into a corresponding 
opening at the back of the fire-box, which is covered by a plate provided 
with a packing-box, through which passes the spindle of the regulator or 
steam valve. By this valve the quantity of steam admitted into the 
cylinders is regulated, and it is constructed in various ways. In the engine 
before us it is what is called a disk valve, consisting of a circular plate, 
from which two segments are cut, working steam tight against a similar 
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