rocker-shaft makes all the difference in the world; in fact, the eccentric stands opposite to what it would without the rocker-shaft. When the engine runs in the direction of the arrow, the eccentric pulls the eccentric rod in the direction

Fig. 30.

of the arrow under it, and drives the valve-stem in the direction of its arrow from the left to the right, as is required. The rocker-shaft just reverses the order of things, that is, it causes the eccentric to be the lap and lead nearer to the pin from a centre line cc, and also follow the pin, while without the rocker-shaft, the eccentric is as seen in Fig. 29, the lap and lead further from the pin, from the line cc, and it also leads the pin.

When the rocker-shaft is used, it should stand at right-angles with the valve-stem, when the valve is over the centre of the seat.

ENGINE PROPORTION AND CONSTRUCTION.

WE shall endeavor to give our ideas of proper construction of the detail of modern horizontal engines, requisite in the machine for the faithful performance of its work as a motor for large manufacturing establishments. A stop of thirty minutes, caused by hot journal slides, or any part of imperfect design, entails a loss to the owners of from thirty to fifty dollars. It cannot be denied that this trouble is constantly occurring, and several cases have come to our notice within the past six months-one in particular, that of a five hundred horse-power donble engine in constant trouble from hot slides, and will continue to be so until the surface on cross-head gibs is made equal to the labor, or according to any of the well-known rules on the subject of friction and lubrication. Then the water pans (we would suggest a more appropriate name— "bath-tub"), can be removed from the slides, and the owners of this costly machine can settle down to business without fear of from two to five stops a week. The slides and cross-head are not the only seat of trouble when treating of

wearing surfaces is considered. The hot crosshead pin and crank pin are two common evils that our practical engineers deal with often, and seven out of every ten cases are due to bad proportion at these vital points of transmission, the trouble being that the builders make their crosshead pins not only too small in diameter, but entirely inadequate as to length. Now when Clear the cross-head is properly guided, and provided with complete adjustment, this pin should be made one and one-half times its diameter in length. The pin itself should be made separate from the cross-head proper, then as soon as a flat or oval form from the oscillating movement becomes perceptible, the pin can be turned onefourth the way round in cross-head, thus offering a new and perfect surface to receive the wear.

By making the pin long, we approach as near as possible (with a single end rod) the old forked connecting rod, still in use in Europe, and always free from heating.

The troublesome crank pins are, on the other hand, the longest ones. The great mistake is made in having too much distance from centre of strain to edge of support. Most builders make their crank pins from one and one-quarter to one and one-half times the diameter; and of course the long distance from crank face causes the pin to spring, and every sixteenth of an inch in length aggravates the disease. A crank pin running on

a hundred horse-power engine, its length being twenty per cent less than the diameter; at the same time by the increased diameter the surface is ample for one hundred and fifty horse-power. The pin was never hot-but the trouble of all troubles comes when the heating selects the main crank shaft journals for its point of attack; this is liable to occur on any engine of size from the moment it makes the first revolution to the time it is ready for the second hand machinery dealer. Not always from badly proportioned journals and boxes, but from the fact that not one in ten of our engine builders provides any vertical adjustment for the crank shaft-it will wear down from its own weight, and that of the pulleys or fly wheels attached to it, and no man living ever saw both ends go down alike. Almost invariably, after three months running, the back end will be the lowest from one-eighth to threeeighths of an inch, most of this being due to the settling of the foundation. When the shaft leaves its horizontal, not only does the heating reach both main bearings, but of course the horizontal of the crank pin is also affected, and it at once falls into line and makes the agony complete, when twenty-five dollars extra expense on each box, in the way of a screw or wedge vertical adjustment, would place the shaft on its original level plane without loss of time or damage of any kind.

In

There are many good devices for taking up the wear of connecting-rod boxes, but the good kinds of adjustment are not the cheap ones. the first place, the adjustments should be so arranged that the length between centre of pins will always remain the same. This assures the least possible amount of piston clearance in cylinder without danger of striking heads; then the taking up of wear should be accomplished by a gentle and moderate motion, and not by sledg. ing on a key by main strength, thereby springing and otherwise injuring the brassess.

The moderate adjustment is not only safe on the brasses but safe in its working; it is never liable to drop out while the engine is in motion, and this is an item of vast importance with the new rapid motion machines. The weight of connecting-rod at centre and at crank end requires the most careful consideration, but is slighted too often in this respect. We have seen a rod on a one hundred and fifty horse power engine 20×48, making sixty-five revolutions per minute, springing by actual measurement

of an inch at the centre at each downward throw of the crank, making the necks of this rod liable to fracture at no distant day. Not counting the injurious effect on the lower slide, twenty-five per cent. less weight in this case would have given ample strength.