rather that diagrams always assist elucidation. Expansion produces economy, and the lap and lead of the valve, in propor

tion to the stroke of the engine, govern expansion.

It has been stated that by the introduction of two ports in the place of one, a reduction of one half of the travel is attained. Now, in order to still further reduce the stroke of the valve, three ports are sometimes adopted. It may be argued that by this arrangement the steam will be wiredrawn. In practice, however, this presumed evil is not apparent, and an almost perfect indicator diagram has been taken from cylinders fitted with three-ported valves. The illustration, Fig. 1, represents a valve arranged by Mr. C. Sells, chief draughtsman to the firm of Messrs. Maudslay, Sons, and Field. The valve is depicted

at full stroke, hence the relative proportions of

F

the means for the supply and exhaustion can be readily understood. Valves, thus designed, have been fitted to many vessels with good results, in fact almost equal to the most perfect expansion valve and gear.

The illustration, Fig. 2, represents a slide valve and casing, as designed and constructed by the firm of Messrs. J. and G. Rennie. This example is perhaps the most graceful sectional form that can be conceived. The illustration being complete, further description will be superfluous.

CHAP. VI.

THE DELINEATION OF THE PATH OF THE CRANK PIN.

The motion imparted to the slide valve is generally derived from two principles of action -vibratory and rotary. Now, when the former is the prime mover, the speed of the valve is the same throughout the stroke, or rather, if the motion is imparted by the piston, the motion of it and the valve would be equal. Rotary motion is more often adopted than any other for the transmission of power and action, and to the present day the small cranks and circular eccentrics are the prevailing means employed to impart the motion required for the slide valve. The speed of the crank and the eccentric are proportionately the same in theory and practice. The length of the connecting rod in all examples of rotary motion regulates the inequality of the speed of the sliding body; it must be remembered, however, that a reciprocity of motion can be attained by correct distribution of detail.

Before proceeding further with the definition of the better arrangement it will not be out of place to define the relation of the path of the

crank to the sliding motion imparted to the valve, also the controlment of the steam. It is doubtless universally known that, virtually, the crank path is divided into four distinct parts, also that for the eccentric. The proportions of these divisions are practically regulated by the grade of expansion agreed on to be maintained. Fig. 1 represents a crank path, the chords indicating the

FIG . 1.

2.

4

divisional points, the relative proportion is therefore, readily understood. No. 1 is the chord of supply, No. 2 that for expansion, No. 3 relates to exhaustion, and No. 4 represents neutrality, or that portion of the stroke of the piston when the port on the exhaust side is covered, often termed compression, consequently, the piston for a period is devoid of pressure or vacuum. The length of the chord 1 is due to two causes, the grade of expansion and the length of the connecting rod. It will be noticed that the chord at the plane line