The conditions as to the steam valves for blowing engines are no different from what they are in any other type under similar conditions of size, speed, and economy desired. It is therefore obviously unnecessary and undesirable to go into an extensive discussion of this subject here.

In American practice the Corliss valve has won its way to almost exclusive use in high-grade steam engines. The poppet valves, which have given magnificent results in Europe, have never become popular in this country in spite of the economical results which have been obtained with them. It is probable that the conditions of operation are somewhat more exacting with that type of valve on account of its coming against a rigid seat, than they are with the Corliss valve with its rotating motion and absence of sudden stops. At all events the result is as stated, that the Corliss valve gear is preeminent in all the modern blowing engines in service and being built in America at this time.

The original Corliss valve gear was the subject to the disadvantage that if the cut-off did not occur at a point somewhat before half stroke it did not occur at all, and steam followed full stroke with utter destruction of economy and terrific stresses on the engine.

With engines on constant mill loads, and particularly before the introduction of the compound which divided the expansion between two cylinders and greatly retarded the point of cut-off in each, this was generally not objectionable, but with the relatively late cut-off necessary for the compound engine, the variable loads to be met, and variations in the steam pressures owing to irregular conditions, so limited a cut-off was not satisfactory for blowing engines, so that for several years makers have been adding an eccentric which enables cut-off to take place under the control of the governor at any point up to about eight-tenths stroke in the high-pressure cylinder, and this constitutes a great advantage under operating conditions, so much so that no engine should be installed for furnace work without it.

Theoretically the low-pressure cylinder does not need the long range cut-off so much, but practically the necessity of operating even though steam pressure be down, and the low-pressure cylinder doing most of the work, makes it very desirable to have this gear on the low-pressure cylinder also, and personally I should not recommend any furnaceman to buy a blowing engine not so equipped.

This feature is also of great advantage in starting an engine up from any position in which it may stop, without barring it over, and in a heavy engine this is a convenience well worthy of the expense.

The resultant diagrams, Figs. 91, 92, 93 and 94, indicate certain features as to the stresses in engines which are worthy of attention. It is proper to say that engineers are by no means unanimous on some of these points, and for that reason the views which I shall express are simply my own and are undoubtedly not in accordance with those of some others well qualified to pass on the subject.

There will be seen at the left of Fig. 91 a sharp point extending about 20 lb. per sq. in. below the line of zero pressure at the beginning of the stroke, which runs down to nothing at about 5 per cent. of the stroke. This pressure comes from the re-expansion of the air left in the clearance space of the air cylinder, and, of course, is in the same direction as the pressure of the steam in the steam cylinder at that time. As soon as the pistons have moved a short distance this air re-expands down to atmospheric pressure, and thereafter the rising pressure in the other end of the air cylinder is increasingly opposed to the steam pressure. It will be seen that if the steam reached full pressure in the steam cylinder at the moment the stroke started, as is customary and proper in power-engine practice, there would be practically double pressure on all the reciprocating parts and on the engine frame at this point, at which the velocity of the piston for a given amount of crank travel is very slow, so that this excessive pressure would come at a time to produce the least useful and the most fric-tional effect.

Inspection of the diagram, Fig. 73, will show that the engine builder has prevented this in part by delaying the admission of live steam so that it does not reach full pressure until after the piston has moved an appreciable distance, by which time the pressure of air in the clearance space is considerably diminished by re-expansion. For this reason the combined net pressure diagram of air and steam cylinders (Figs. 92 and 93) instead of showing a sharp "spike" on the left-hand end of the diagram, as it would if steam were admitted to full pressure at the beginning of the stroke, shows only a considerable "hump." The practice in the case of the low-pressure diagram of the same set (Fig. 75) is even better in this respect, for it will be seen that in this case the piston has traveled the whole distance necessary to permit complete re-expansion of the air before steam reaches full receiver pressure in the cylinder, and if a combined diagram were made for these cylinders the hump at the left, shown in Figs. 93, 94 and 95, would not appear.

Following through this same idea these builders eliminate any compression whatever, both in the high-pressure and low-pressure cylinders. In fact, there is nothing in either diagram to show that the steam first admitted at the beginning of the stroke may not blow through into the exhaust. It is not possible to say from the indicator diagram that it does so, and the care and good judgment of the builders make it unlikely, at the same time so considerable an economy of steam may be effected by very little compression, and with such a slight increase in pressure, that I consider a small amount of compression desirable both for the steam it saves and to make it absolutely certain that live steam does not blow through into the exhaust at the beginning of the stroke.