338 DETAILS OF (PADDLE-WHEEL)
cap of the spring, while in other instances itforms part of the same.
Relief valves of another design are shown,located on the top and sides of the cylinder,by Fig. 55—page 225. In that instance eachspring is retained by a cross bar and side rods ;the valves—of the disc kind—are sustained asnear the facings on the cover - and cylinder aspracticable, thus dispensing with the projec-tions cast with the body, as shown by Fig. 68—page 237. In the event of instantaneousrelief, plug valves are often introduced at thelower ends of the cylinders, and the plugs areconnected with handles secured at the top end,also shown by Fig. 55—page 225.
The following illustration, Fig. 69 , is therepresentation of a connection with the topand bottom of the cylinder, the lower plug
CYLINDER RELIEF PL0G VALVE.
forming an exit from either locality. The topplug is surmounted by a handle, and thus anymotion imparted to it is transmitted direct tothe lower plug. To ensure this, the connec-
tion of the plugs is a subdivision, and thus acertain action is rendered by the mode ofcontact. It will be noticed that the plugs areshown both open, but should a contrary effectbe required, it can be readily effected byreversing their positions at the connection.This mode has been adopted by ourselves— and doubtless by others — with muchsuccess.
The remaining details of the type of engineunder notice consist of matters of circum-stance rather than general design. Forinstance, the injection valve gear may beof a given arrangement in one example andtotally different in the next. The same canbe stated of the gear for regulating theadmission of the steam, usuallv termed thethrottle valve gear. The information respect-ing the hand pump, water discharge valve,donkey engine, and other connections, beingas those for screw engines, attention respect-ing the same to that section is directed.
PADDLE WHEELS.
The means of propelling vessels by thesliding action of a flat substance interceptingthe water, is the original mode adopted bythe most uncivilized race of mankind. TheIndian, with his paddle, performs feats ofdexterity, not only in speed, but also indirecting his frail bark across the most in-tricate and dangerous routes. It is knownalso, that, before the use of machinery, therowers of a ship formed an important massof the crew. Now, simple as it may seem torow, or handle a paddle, there is, however,much skill to be exercised in producing thegreatest effect from the least cause. Evi-