so as to leave shallow recesses about inch deep, reduce the actual surface which touches the guide to about 5% of the original area; there will be then strips across the slipper 14 inches wide, with depressions between them 34 inch wide, filled with grease.

Cast iron, hard and close grained, is the best material for the guide-plates; its surface, after a few hours' work, becomes exceedingly hard and highly polished, and offers very little resistance to the slipper or guide-block. So long as this hard skin remains intact, no trouble will be experienced, but if abrasion takes place from heating or other cause, it rarely works well after, and should be at once planed afresh.

Guide Plates.-In order to have a sound and hard surface for the piston-rod slides to work on, the guide-plates should be separate from and secured to the columns; when so fitted they also admit of adjustment, and may be cast hollow, so as to permit of a flow of cooling water through them. This is especially needful for large quickrunning engines, where the speed of piston is very high, and any want of lubrication would soon cause most serious damage. Cast iron when once worn smooth gives a splendid surface for a slide; but if by any mischance this surface. suffers a little abrasion, it is most difficult to get right again, and will seldom work well again until it is.

The face of the guide plates should have good oil courses cut on it, so that the lubricant is well distributed, and they should be cut deep enough to prevent their being choked with the gluey deposit from the oil. The piston-rod slide should always be provided with a comb, which will carry the lubricant from the drip-boxes, and spread it over the face of the guide.

Area of steam ports and of section through passages

(Diameter of cylinder)2 × speed of piston
7636

Opening of Port to Steam.-It is advisable so to design the valve, &c., that the opening for admission of steam to the cylinder is sufficient to avoid any serious loss by "wire-drawing;" but in actual practice, unless special gearing is designed so as to give a quick motion to the valve at the instant of cut-off, there is very considerable loss of pressure shown on the indicatordiagram; and, what is worse still, from deficient opening, the loss is generally not limited to the period of cut-off, but during the whole time of admission. The ordinary valve-gears do not give that quick motion, either at opening or at cut-off, which is such a desideratum. Separate expansion valves and special valve-gearings admit of such a motion, and consequently the

opening to steam with them may be smaller than when cut-off is effected by the ordinary slidevalve and link-motion.

Hence, when only common valves and gear are to be used, the area of opening to steam when at its greatest should be such that the mean velocity of flow does not exceed 10,000 feet per minute. When expansion valves, or special valve-gearing, is used, the mean velocity may be assumed at 12,000 feet, although it is better to give such an amount of opening, when possible, that the velocity shall not exceed 10,000 feet. In actual practice the amount of opening is often much less than that given by the above rules, but it always results in loss of pressure in the cylinder throughout, and excessive "wire-drawing" previous to cut-off.

Exhaust Passages and Pipes.-The area of section of exhaust passages should be such that the mean velocity of steam does not exceed 6000 feet per minute, and if the distance from the cylinder to the condenser is comparatively great, a somewhat larger area is advisable. There should not be a greater difference than 1 lb. between the pressure in the cylinder and that in the condenser when exhausting.

Receiver Space. The space between the valve of the high-pressure cylinder and that of the low pressure cylinder, into which the steam exhausts from the high-pressure cylinder, should

be from 1 to 1.5 times the capacity of the highpressure cylinder, when the cranks are set at an angle of from 120° to 90°. When the cranks. are opposite or nearly so, this space may be very much reduced. The pressure in the receiver should never exceed half the boiler pressure, and is generally much lower than this. It is usual to fit a safety valve to the receiver, loaded by weight or spring to a pressure of 20 to 30 lbs. per square inch; otherwise, owing to the large flat sides between the two cylinders, great risk of explosion would be run. This safety valve is usually of the same size and design as the cylinder escape valves. The receivers of three-crank engines need not be nearly so large, as the cranks are usually at angles of 120°; in the case of triple compound engines with the M. P. leading the H. P., a very small receiver will do.

CONNECTING RODS AND BRASSES.

The following empirical formula will be found a very useful one, and the results given by it agree very closely with good modern practice.

Diameter of connecting-rod at middle

=

VLXK.
4

L is the length of the rod in inches, and

K = 0.03 X V Effective load on the piston in lbs. Example.-To find the diameter of the con

necting-rod, 100 inches long, for an engine having a load of 55,000 lbs.

The diameter of the connecting-rod at the ends may be 0.875 of its diameter in the middle. The tapering of rods, or making them barrelshaped, is usual in the case of those having single brasses at both ends, such as are generally fitted to trunk and return connecting-rod engines; then the diameter of the crank-pin end is 0.925 of the diameter at middle. Direct-acting engines have usually the connecting-rods tapering from the gudgeon end to the middle, and then parallel or nearly so to the crank-pin end.

Connecting-Rod Bolts.-The diameter of the bolts may be calculated by allowing the same strain per square inch as that given for pistonrod bolts. It is usual now, from practical considerations, to make the bolts of both piston and connecting-rod of the same size; the bolts therefore should be calculated from the strain on the connecting-rod. In order that the whole of the stretch shall not come on one section, as at the bottom of the last thread of an ordinary bolt, it is better to turn part of the body of connecting and piston-rod bolts to the same diameter as at the bottom of the thread, leaving it a little larger