port openings at the various points of the stroke are measured by the height of these curves over and under the opening edges of the ports at both ends of the valve, respectively.

It will be noticed that these ellipses are slightly flattened on one side, which is caused by the slower lineal motion imparted to the valve relative to the angular

EXTERNAL ADMISSION.
I" LAP
LEAD

EXHAUST LINE & LINE

FIG. 32.-Long's Diagram of Valve Events Effected by Walschaerts' Motion.

motion when the eccentric passes its back centre compared with that of the front centre, due to the angularity of the eccentric rod, and is more marked the shorter the rod. Fully symmetrical ellipses are not obtainable, as this would require the eccentric and main rods to be of infinite length; this angularity, however, is of but little detriment to the distribution

of the steam as long as the relation between the length of the eccentric rod and the throw of the eccentric is not less than the given limitations, and is present in all kinds of continuous valve motions derived from uniformly rotating cranks or eccentrics.

GENERAL NOTES FOR ADJUSTING WALSCHAERT GEAR.

1. Ascertain by the following method the position of the eccentric crank: Mark the position of the link relative to its middle position on both of the deadcentres of the main crank. If the position of the link is the same in both cases, the eccentric crank position is correct; if not, the eccentric crank should be shifted until this occurs, or as near so as possible.

2. After the eccentric crank has been correctly set the eccentric rod should be lengthened or shortened as may be required to bring the link in its middle position so that the link-block can be moved from its extreme forward to its extreme backward position without imparting any motion to the valve. It may be noted that the link position may be observed by the usual tram marks on the valve-stem, or direct by marks on the link pin, as may be found most convenient with the link-block in full gear-preferably ahead.

3. The difference between the two positions of the valve on the forward and back centres of the engine is the lap and lead doubled; it is the same in any position of the link-block and cannot be changed by changing the leverage relations of the combination lever.

4. The tram marks of the opening moments at both ends of the valve should be marked on the valve-stem and the latter lengthened or shortened until equal leads at both ends are obtained.

5. Within certain limits this lengthening or shortening may be made on the radius rod if it should prove more convenient, but it is desirable that its length should be so nearly equal to the radius of the link that no apparent change in the lead should occur in moving the link-block as stated in note No. 2.

6. The lead may be increased by reducing the lap, and the cut-off points will then be slightly advanced. Increasing the lap produces the opposite effect on the cut-off and reduces the lead the same amount. With good judgment these quantities may be varied to offset the irregularities inherent in transforming rotary into lineal motions.

7. The valve events are to a great extent dependent on the location of the suspension point of lifter of the rear end of the radius rod, when swinging lifter is used, which requires that this point should be properly laid

out by careful plotting, or, if convenient, it is preferably determined by a model, as irregularities due to incorrect locus of this point cannot be corrected by the other parts of the gear without more or less distortion of same. When this point is so fixed that a change of same is impracticable it may be better, however, to modify other elements if thereby the motion in general can be improved.

HELMHOLTZ MODIFICATION.

Among the various modifications of the Walschaert gear the one made by Helmholtz is probably of some advantage. This modification consists in making the link straight, and the radius rod is connected to the lifting link instead of to the link-block. The curving of the link is compensated for by the reversing shaft, or lifting-arm fulcrum, being located in a given. position above the link, so that the locus of the suspension point of the lifting link forms an arc of a circle with its chord perpendicular to the centre line of the radius rod in its centre position. The radius of this arc bears the same relation to the length of the radius rod as the distance of the radius-rod connection above the link-block bears to the length of the lifting. link, which results in that this connection is moving in

an arc with a radius of the length of the radius rod, and the same motion of the valve is obtained as in the direct Walschaert gear.

Two advantages may be claimed for this modification, of which one is the straight link being simpler to make than the curved one, and the other is that on large piston-valve engines with inside admission the link fulcrum can be lowered by the amount the radius-rod connection falls over the link-block, whereby the eccentric-rod connection can be brought closer to the centre line of the axle with less length of link and eccentric throw. It has, however, the disadvantage that there is little choice in the location of the reversing shaft, or lifting-arm fulcrum, a proper position for which is hardly obtainable on all types of engines, and admits of no other method of lifting the radius rod in linking-up, or reversing, the engine.

Fig. 33 shows a combination of two diagrams: namely, those of Reauleaux and Zeuner, which coincide exactly as to the different valve events, which may be found as follows:

The distance AB represents the travel of the valve as well as the stroke of the engine, though in different scales, which makes no difference when the cut-off is always expressed in fractions or per cent of AB. The maximum cut-off is determined upon to be AR. Draw a perpendicular line RC from AB until it cuts