28 TECHNOLOGY. 
out the necessity of uncoupling the eccentrics; all that is necessary being 
to change the lever 1’ from one extreme of the arc in which it moves to the 
other, which, through the levers &' /’/, so changes the position of the slotted 
link that it either lies at its top or bottom, and receives motion from either 
one or other of the eccentrics. If the lever 1’ is in a vertical position, then 
neither of the eccentrics will predominate, the slide-valve will remain 
stationary, and no steam will be admitted to the cylinders. The working 
by expansion is effected in the following manner. In the vertical position 
of the lever 1’, although the driving-shaft may continue to revolve, no steam 
is admitted to the cylinder; when, however, this lever is at either extreme 
of its motion, the valve-rod makes its full stroke, the valve opening its 
entire passage to the cylinder ; consequently the nearer the lever 1’ is to its 
vertical position, the shorter will be the stroke of the valve-rod, and the 
sooner the steam will be cut off and permitted to act by expansion. Thus 
the engineer has it within his power, by operating the lever 1’, to work his 
engine with any degree of expansion, forwards or backwards. The feed 
pump is at p, fig. 1, and in detail fig. 5; jig. 2 shows the arrangement of 
the tubes in the boiler. 
Of the numerous improvements which have been made in locomo- 
tives, we will mention only one, which has caused quite a revolution in their 
construction. William Norris, of Philadelphia, ran the connecting-rods to 
crank-pins in one of the arms of the driving-wheels, in place of attaching 
them to cranks upon the axles. The advantages of this modification are 
very great, and most of the European locomotives are now built upon this 
plan. Inthe Norris locomotive the cylinders, with their valve chests, lie 
outside of the main frame upon both sides of the boiler. By the whole 
arrangement a double advantage is gained. Where the crank was upon the 
driving-axle it described a circle equal to the half stroke of the piston, 
added to the thickness of the crank-wrist ; consequently the stroke could not 
be lengthened without raising the boiler. This is not now necessary, and 
the diameter of the boiler may be increased, and consequently the number 
of the flues and the extent of heating surface, as well as the length of stroke 
of the pistons. The axles being now perfectly straight, are stronger and 
more durable with the same weight of iron. The working parts are more 
exposed to view, more easily controlled, and in case of injury or accident 
are more easily repaired. | 
b. Passenger Cars. Railroad cars require an entirely different construc- 
tion from ordinary carriages to enable them to run with the necessary 
velocity and safety. We shall notice first the wheels and axles, then the 
trucks, brakes, and couplings. The wheels and axles are of primary 
importance both as regards the safety of passengers and the durability of 
the cars. Constant efforts have been made to obtain a cast-iron car-wheel 
without flaws, and of sufficient strength to withstand the shocks to which it 
is exposed ; but it would seem that this has never yet been accomplished, 
although the desired end has doubtless been approached. It is necessary in 
the first place that the rim or tread of the wheel be very hard, otherwise 
it would soon wear uneven from constant contact with the rails, and become 
608 
