348 THE PRINCIPLES OP THE MARINE ENGINE.

travel forth until the inside edge permits exhaustion ordestroys expansion. The valve is now at half stroke,plus inside lap; exhaustion of steam, therefore, mustensue until the valvo is in the same position, but travel-ling in a reverse direction. The position of the valvewhen terminating exhaustion will be half stroke, minusinside lap. It can thus be clearly understood that thelength of the chords for expansion, and that for compres-sion, are equal in the example given; it may also be addedthat any variation in these two chords will depend onvarious causes, such as unequal laps and leads, &c., &c.,but with certain arrangements both are equal. The factof the compression being the same as the expansion, is ofno vital importance. It is certain there would be a gainin maintaining expansion longer, and exhausting, tillthe supply commenced, and thus dispensing with com-pression ; but the present motion of the slide valvewould have to be altered, as an extreme unequal actionwould have to be attained.

The proportions of the path of the crank pin canthus be clearly understood. As at present arranged—compression, supply, and expansion, form one portionof the circle, and the remainder is occupied by ex-haustion, which is the most. The diagram givenrelates to one revolution of the crank only, it being wellknown that all in principle are alike. There is, how-ever, a slight variation in practice, due to the versedsines of the rods, and inequality of the length of arcspassed through, in proportion to the propelling or sHdingmotion attained.

The arrangement of the slide valves, in relation to thatof the connecting-rod, should be considered as to theattainment of equal action. The versed sines of thechords of the arc passed through by the crank pin, ateach angle of the stroke, for a given supply of steam, areunequal, also tho versed sines of the eccentric, when theslide is at the same side of the crank as the connecting-rod;but this can be counteracted to a certain extent by shorteccentric rods, and levers, reverse in action, and arrangedto compensate for the inequality of the speed of thecircular and sliding motions, the attainment of whichbeing of great importance.

Fig. 232 represents the circle described by the crankpin for a given stroko of piston. The horizontal line ispresumed to be the centre of the engine, and the largerdotted arcs represent an even grade of expansion, oneach side of the piston or at each stroko. Now it willbe seen that on the crank—represented by the thicklines—moving from tho plane to the intersection at A,

a given length of stroko of piston is produced, due, ofcourse, to the radius of the larger dotted arc. On thecrank reaching to the intersection at B, the grado ofexpansion is reversed in action, but the same distance

Fig. 232.

VE EFFECT BT THE POSITIONS OF

BURGHS DIAGRAM OF THE RELATJ

THE SLIDE VALVE AND CONNECTING ROD.

from tho end of the stroke retained. Now, the differ-ence in the lengths of tho arcs passed through are dueto the length of tho dotted arcs, the radii of which arethe connecting-rod. The smaller circle a indicates thetravel of the valve, or the path of the centre of forma-tion of the eccentric. The dotted arc indicates tho dis-tance the valve must be from the edge of the port, whenthe piston is at full stroke, hence the angle or advanceof the eccentric to that of the crank when on tho hori-zontal line. When the crank is at A, the eccentric haspassed through an arc equal in length proportionately-The two dots on the circle opposite the dotted arc indi-cate the angles of the eccentric when the crank is at theplane line and at B. Now it wiU he readily seen thatthe space between the dots is less than that of the in-tersections opposite; also the versed sine reduced. Itis obvious that an unequal grade of expansion mustensue on the side of the circle at B., i.e., if the laps ofthe valvo are equal. It must also be remembered that,to increase the lap and retain the previous stroko of thevalve is to destroy the lead; hence, the slight variation inthe time for supply steam at each stroke of the piston maybe said to be endured rather than introduce a worse evil.

It is not here intimated that perfection of mechanismshould not be attained, but rather to delineate and de-scribe that most universal. The diagram now underquestion, illustrates the principle of the action of thecrank and eccentric with the valve situated on the sameside of the crank shaft as the connecting-rod, directaction, in each case being maintained. It will be seenthat the arcs are all struck by the radii on the sameside of the perpendicular line, hence the variation abovealluded to. Now in order to counteract, or rather oh-