rod attached to the combination lever in such a manner that the angle given to the lever by the piston being at the extreme end of the cylinder has moved the valve from an otherwise central position a slight distance in an opposite direction from that occupied by the piston, as is required in order to open the proper admission port for lead.

FIG. 28.-Comparative Diagram of Combination Levers Used with Outside Admission and Inside Admission Valves.

To give lead to the valve with inside admission the valve-stem and radius rod are connected to the lower combination lever in an exactly opposite manner to

the

way in which the same connections are made on the other combination lever just described. The angle of the combination lever in this case pushes the valve in the opposite direction, and the reason for this is plainly

seen by reference to the plate: steam admission past the valve takes place externally with the upper valve and internally with the lower one, yet in each case the steam is admitted to the same end of the cylinder, and this requires that the two kinds of valves be advanced in opposite directions.

The Mason Machine Company undoubtedly were the pioneers in the attempt to introduce the Walschaert valve gear into this country, and if that company was still in the field, their experience would help greatly in the present-day application of the gear. The American Locomotive Company and the Baldwin Locomotive Works are, however, making a specialty of supplying the Walschaert motion to their heavier engines, and the Baldwin Company gives the following instructions for erecting the gear and setting the valves:

1. Check carefully the dimensions of the following parts, rejecting any that are not exactly to drawing: a Valve

b Valve-stem.

c Valve-stem crosshead, or slide.

d Combination lever.

e Crosshead link.

f Link radius rod.

g Reverse link.

h Location of combination lever on crosshead.

k Length of eccentric crank.

2. Check eccentric throw to see that it is exactly as specified.

3. Be sure that guide-bearer is correctly located from centre of cylinder, as the reverse link is usually attached to it, and variation in the location of the link cannot be allowed. If the link is attached to separate crosstie, similar precautions must be taken to insure its correct location.

4. Exercise great care in the location of the unk, so that the trunnion (fulcrum) centre is exactly to dimensions from the centre of cylinder.

5. See that the reverse shaft centre is correctly located to dimensions given, and that the lifting arm and link are of the exact lengths as specified.

6. Connect crosshead gear to valve, and radius rod to link, without connecting eccentric rod to link.

7. Hook up radius rod to exact centre of link, and then revolve driving wheels, seeing that crosshead gear gives correct lead as specified for both front and back admission ports.

(Say that inch is the required lead: with the steam chest open at this time-in the case of a D-slide valve -it should be seen that when the crank pin is at either dead-centre the admission port is open of an inch. With outside admission valves this will be plain, but with piston valves of inside admission a very little steam or compressed air can be used, and by marking the

valve-stem and noting the blow from the cylinder cocks and the point at which the pressure ceases to escape, the amount of lead can be closely approximated. It should be possible, however, to obtain correct results in this phase of the motion with the eccentric rod connected with the link, for it is required that when the reverse lever is at mid-gear the radius rod shall be centred in the link, and the motion of the latter shall have no influence on the valve nor combination lever.)

8. Connect link to return crank by eccentric rod, and obtain full travel front and back, and in both forward and backward motions, correcting any errors by lengthening or shortening the eccentric rod as previously noted.

The valves may now be considered as definitely set, and may be tested to any cut-off points in the usual

manner.

A simple additional check should be made as follows: Set one side of the engine so that the piston is at its extreme forward position in the cylinder, and check lead on the admission port.

In this position it should be possible to move the link-block through its entire travel in the link without in any way disturbing the movement of the valve.

This operation should then be reversed and the other side of the engine similarly tried with the piston located at its extreme backward position in the cylinder.

If an inspection of the Walschaert gear reveals an error it must not be regarded as a constitutional fault that cannot be corrected; go over the motion carefully and it will not be hard to find the cause-a cause that probably can easily be adjusted; for, while it must be realized that as it is impossible to perfectly "square the circle," and is as equally impossible to exactly convert the circular motion derived from the driving wheel or its axle into the straight-line motion of the valve without error-especially when the results from different points of cut-off are required to be the same in forward gear as with the engine reversed, in back gearyet the designers have found means by which those inherent irregularities may be practically dissipated and the errors so far reduced that after the natural wear of the working parts has been taken up they cannot be detected, nor will they lessen the economy or power of the locomotive. It must not be inferred, however, that the foregoing statements are made to excuse any inaccuracies that may exist within Walschaert's type of valve gear when turned out of the builder's hands that render it inferior to the Stephenson motion as to the cut-off points, etc.; the Walschaert motion leaves the shop with an indication of truer locomotive valve performance than any other, excepting, perhaps, the Allan gear, which is generally regarded as the most perfect valve motion for loco