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M.A. RR. LN OS | pressures, which accordingly resulted with a consequent reduc- tion in the weight of machinery per unit of power. - An accompaniment of the introduction of surface condensa- tion, which was at first supposed to be a result of it, but which as a matter of fact was not, was a tremendous increase in the corrosion of the boilers and shortening of their life. This was especially noticeable in the tubes which, as the thinnest part of the boilers, gave out first. All sorts of theories were advanced to account for it, some of which we can now see to have been utterly ridiculous. Probably one of the most fanci- ful was that which regarded the boiler and condenser as forming a gigantic galvanic battery, the copper condenser tubes forming one pole and the boiler the other. The real facts were developed as a result of the investigation by the admiralty committee on boilers in the 70's which showed that boiler corrosion was simply rusting and had been due to gross but unintentional neglect. It had been a very common practice, particularly in naval boilers, when they were not in use, to blow out the water, and take off the manhole plates "to let them air." It was this "airing" which caused the corrosion. Now when boilers are laid up, they are filled with water which is made slightly alkaline, and this effectually prevents cor- rosiort. THE CYLINDRICAL BOILER. The early boilers in sea-going vessels were of what has been called the "box" type; that is, the boiler was a cubical box with a thin shell, the real strength being given by braces running in three directions. When surface condensation had made higher pressures possible, it was soon found that the multiplicity of braces, as pressures were increased, made an impossible condition of affairs, and this led to the design of the cylindrical boiler whose shell was self-bracing and left the only braces, those needed for the heads and flat surfaces. This boiler so thoroughly met the conditions arising that it has remained the favorite even up to the present day. At one time an effort was made to save room by making the boiler ellipti- cal, but this was soon found to be unsatisfactory and impracti- cable and was abandoned after only a few examples. The éarliest cylindrical boilers were single-ended with two furnaces, but with the advent of reliable mild steel the diam- eters were increased and the boiler was made double ended, with the upper ends rounded to save bracing, so that the largest cylindrical boilers today have as many as eight fur- naces, four in each end in pairs; that is, the two furnaces at each end on the same side of a vertical diameter have a common combustion chamber. The saving in weight due to the double ended boiler is evident at once and also the reduc- tion in the feeding apparatus required. Notwithstanding the advent of the water-tube boiler which will be mentioned further on, and its practical pre-emption of naval practice, the cylindrical boiler still remains the favorite for the merchant service and has been used for pressures as high as 220 lbs., even in the largest sizes on such vessels as the Kaiser Wilhelm. The highest recorded pressure is 253 lbs. on the Inchdune. THE COMPOUND ENGINE. From a very early period steam had been used expansively in marine engines; and indeed sometimes to a ridiculous extent. Some engineers as late as the civil war hardly seemed to realize that there was any limit to expansion, although Isherwood's famous experiments on the Michigan in 1861 had demonstrated conclusively that, with low pressures, only a very moderate expansion is permissible, beyond which any further expansion is attended by an economic loss. As pres- sures increased it was natural and correct that a higher range of expansion should be used, and this made practicable the compound engine where the expansion occurs in two stages, the high pressure steam from the boiler being limited to a small cylinder from which, in turn, the steam of lower pres- R & V7 19 sure is exhausted to a larger cylinder. As you all know, the compound engine was invented almost as early as Watt's separate condenser, Hornblower's patent dating back to 1771, and Wolff's patent for a two-cylinder engine dating back to 1804. With the low pressure prevalent at that time the com- pound engine was actually at a disadvantage compared with the simple one. When pressures had gotten up to about 60 ibs., however, the compound engine began to assert itself, the pioneer in that respect being John Elder of the firm of Randolph & Elder, which is now known as the Fairfield Engine Works. It is interesting to note that the Allan Line of steamers, which is now the pioneer in introducing the steam turbine for an ocean-going steamer, made the last scientific stand against the compound engine, going so far as to take duplicate vessels, and engine one with compound and the other with simple engines of the same power. The actual experience with these two vessels where the simple engine with the high ratio of expansion was constantly in trouble from breaking down, was'a convincing proof that high ratios of expansion in a single cylinder was impracticable. With the improved workmanship which had come by this time and with the improvement in materials, to which we shall refer in a moment, which came later, the compound engine advanced to a high state of perfection and for large powers the three cylinder type, with one high pressure and two low pressure cylinders, became a favorite for all large vessels. These engines were probably as fine specimens of marine engine designing as have ever been seen, and included some exceedingly ingenious valve gears designed to secure variable expansion and an equalization of work among the various cylinders. As we shall see later, the further advance relega- ted these beautiful mechanisms to the engineering museum. THE ADVENT OF MILD STEEL. It is probably difficult for the young men in our technical schools of today who are familiar almost entirely with mild steel and very little with wrought iron, to realize what a change came in engineering when the production of mild steel became a commercially reliable matter. When we look back at the way in which some of the vital elements of a big marine engine were made, we are almost inclined to wonder | that the material was reliable at all. The difference between a large wrought iron shaft such as old Hughey Dougherty used to make at the Morgan Iron Works, and one of the mild steel shafts made at Bethlehem, is as great as could be im- agined. Nearly the same is true of boiler plates. The young engineer of today would hardly know what was meant by a lamination or a "cold shut." The very method of manufacture made it necessary to use a large factor of safety in designing, with the result that the working stresses permissible were very low and the weight of machinery inordinately high. With the. advent of mild steel and the introduction of careful and systematic testing, the designer had a material on which he could place absolute reliance so that the factor of safety could be greatly reduced. As a matter of fact the factor of safety has been reduced from 8 or 10 to 5, and sometimes as low as 4.5. In looking back over my own experience, I do not see how we could possibly have built engines of the size and power now common with wrought iron for piston and connecting rods and shafting, and it is, of course, absolutely certain that we could not have built cylindrical boilers of today. The change began in the later 70's and had become almost com- plete by the middle of the 80's. We must not fail to notice in the change to steel the use of steel castings, which have dis- placed cast iron in many places with attendant great reduc- tion in weight. The first use of steel castings was attended with considerable annoyance because it was unfortunately assumed, perhaps naturally that, barring the much greater shrinkage, it could be treated very much the same as cast iron,