ive and costly experiments, exploded in England and France ten years ago. Engineers then proved that no steam engine would work with. its greatest economy without proper-sized steam and exhaust pipes and ports, quick piston speed and crank-shaft revolution, coupled with liberal compression.

Compress For

It was fully established at the trials and tests just referred to, that it was a mechanical impossibility to reach the desired results in piston speed, crank revolution, and the attending economy of fuel, without a compression on the return to a point in pressure at least three-fourths of the initial. Otherwise the most important details of transmission soon perished, and every part of the engine suffered from undue strains.

We would like to ask the non-compressionist to consider for a moment that portion of the crank circle within 35° of its finish or central plane. Then consider the piston, piston-rod, cross-head, and connecting-rod, moving (at the speeds used in these days) with their foot-pound momentum, due to the velocity and weights, and we will then ask him if nearly all of that . part of the crank's path indicated above, is not wasted energy, without compression?

If he does not utilize the vapor on hand, at this point, he will not only be chargeable with a loss at the cylinder, but will certainly have a terrible account standing against him in the

matter of demoralized cross-head and crankpins, to say nothing of the main journal. Compression has come to stay with us, and we must study to use it to the best advantage. The loss by extra valve leakage, in short stroke and quick revolution, is counteracted by the consequent saving in HEAT.

MAIN CRANK SHAFT AND DRIVING PULLEY.

In calculating the diameter of an engine crank shaft, it is necessary to consider: 1st, the strength to resist torsional strain, and 2d, its stiffness, or what is termed torsional elasticity.

It is well known that a shaft may be strong enough to stand all torsional strains that may reach it without twisting asunder, and at the same time be so elastic from its great length, as to render it entirely unfit to drive any machinery in which steadiness of motion is at all essential. It is therefore necessary to consider the strength of shafts and their stiffness independently, following, as they do, entirely different laws. Torsional strength varies as the cube of the diameter, and is wholly independent of the length, but torsional stiffness varies directly as the fourth power of the diameter, and inversely as the length. Or d1 ÷ L.

Now rules proper for regular line shafting, whether placed in hangers for the ordinary transmission of power, or in larger sizes running in

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bearings bolted to solid foundations, are not proper for engine crank-shafts, owing, of course, to the irregularity of strains upon an engine shaft, due to the influence of crank motion and changing steam pressure.

Having the same labor to perform in foot pounds, the crank shaft must be sixty per cent. stronger than the line shaft when a single engine (one crank) is used; but with a double engine, or two cranks, one on each end of shaft at right angles, an addition to the torsional strength of twenty per cent. will answer, as far as that particular strain is concerned.

Our engine builders thirty years ago used castiron for crank shafts, as a forged shaft above six inches diameter at that time was worth from five to seven times as much as a cast shaft. The majority of the founders cast them solid, the result being that about one-fourth of the sectional area was unsound, and perfectly useless to resist any torsion. Others were wise enough to cast their shafts hollow, or with a core through them, removing fully one-fourth the sectional area, thereby adding strength to the remaining portion of shaft by the superior soundness given to it. The shafts were cast in a pit, on end, of

course.

But the days of cast-iron shafts are passed; we get wrought-iron of the best quality in these days at a price of about double that of cast-iron;

they are much lighter, and are very stiff, from being well-hammered.

Of course the increased speed of engine crank shafts of the present day helps materially their transmitting power; for an 8 inch shaft transmitting one hundred horse power. at sixty revolutions is one thing, and the same shaft carrying the same power at eighty-five revolutions is altogether another "individual."

But it is not a good plan to reduce too much in this direction, for the reason that the momentum of the various machinery driven causes a surging action which creates an excessive and irregular strain, and this must be provided for in fixing the shaft diameter.

In regard to driving pulleys, their size and manner of construction, there is much to be noticed, but we will not now consider the subject at length.

We must here enter a protest, and it is against the practice of speeding-up old driving pulleys twenty-five, and in many cases fifty per cent. above their original speed, for the purpose of increasing power of engine.

They are absolutely dangerous, and will destroy themselves, and perhaps life and property, by bursting-from actual centrifugal force-as soon as the laws relating to the fatigue of metals assert themselves, as they surely will.

Persons who take so much risk seem to forget

that the centrifugal force of these old pulleys increases as the square of the velocity, so that in doubling the speed they have four times the amount of centrifugal force to contend with.

In many cases of these old pulleys the sectional area of metal is able to stand the increase, but there is always a hidden enemy that no man can safely estimate the power of. We speak of the strain brought upon a wheel in casting and cooling, being in halves generally, and consequently large castings. We all have seen apparently sound and beautiful pulleys, from ten to sixty inches diameter, burst asunder in the lathe, the very moment an ordinary cut was started, or by the simple matter of a fall of one or two inches. Even these pulleys, small as they are, are very dangerous when running (if they escape detection in the shop); but to what magnitude is this danger increased, when the diameters and weights are as great as they are in engine driving pulleys.

As we believe the best results from driving belts are obtained at a speed per minute of from 3,000 to 3,800 feet, we do not think it necessary to make any driving pulley for mills or general manufacturing over 14 feet diameter, and in most instances not over 12 feet (piston speed from 600 to 900 feet).

We believe also that all driving pulleys over 8 feet diameter, where high speed is used, should