The report of the Superintendent of motive power, and Supervisor of repairs on the Philadelphia and Columbia road, exhibits a very comprehensive view of the condition and operation of that part of the improvements. His report on the motive power departments presents the following statement:

The number of cars passed over the road in 1850, amounted to 140,394, exceeding the number passed in 1849, by 19,562.

The freight passed over the road amounted to 265,113 tons, being an increase over the last year of 45,632 tons.

The number of miles traveled by passengers was 8,060,278, equal to 98,296 through passengers, being an excess of through passengers over 1849, of 8646.

The number of trips run by locomotives was 8074, or 630,084 miles, being an increase of 604 trips, or 47,112 miles over the preceding year.

General Rules for Proportioning the Length of Boilers for Stationary Engines.*

RULE 1.-A plain cylindrical boiler, without any inside flue tube, and hung on what is sometimes called the "oven plan," that is, with a direct draft passing from the fire-place directly under the bottom of the boiler to the vent or chimney, and without return flues of any kind, need not exceed in length six times its diameter; and it ought not to exceed six times the square root of the area of the fire grate in feet if worked with Lancashire, Derbyshire, or Yorkshire coal, which is equivalent to six times the square root of the nominal horse power of the engine in feet. And if worked with the best Newcastle coal, the boiler need not be more than about eight diameters long, and ought not to exceed eight times the square root of the area of the fire grate in feet, which is equivalent to eight times the square root of three-fourths of the nominal horse power of the engine in feet; and in any case it never ought to exceed six feet in diameter.

RULE 2.-A boiler without any inside flue tube, and set up in the common way, with external brick flues and a wheel draft, need not be more than about four diameters long, and ought not to exceed in length four times the square root of the area of the fire grate in feet for Lancashire coal. If worked with Newcastle coal, it ought not to be more than about five diameters long, and need not exceed in length five times the square root of the area of the fire grate in feet; and in either case, whether a wagon or cylindrical boiler, it never ought to be more than six, nor less than four feet in diameter.

RULE 3.-If a boiler contains one or more inside flue tubes, passing quite through it, and is to be set up with a split draft, it need not be more than about three and a half diameters long, and ought not to be longer than three and a half times the square root of the area of the fire grate in feet for Lancashire coal; and if worked with Newcastle coal, it need not be more than about four and a half diameters long, and ought not to exceed

From the London Artizan, for January, 1851.

four and a half times the square root of the area of the fire grate in feet; and never be less than five feet in diameter.

RULE 4. When a boiler contains one or more internal flue tubes, with an inside uptake or connexion with the boiler bottom, as in Boulton and Watt's, or as in the marine "tubular," and other multiflue boilers, the length of the boiler need not be more than about three times its diameter with Lancashire coal, and ought not to exceed in length three times the square root of the area of the fire grate in feet; but with Newcastle coal its length in feet may be equal to four times the square root of the area of the grate, and need not be more than about four times the diameter of the boiler.

RULE 5.-Cornish boilers and Butterly boilers, set up in the best manner, with split draft, and using Lancashire coal, need not be more than about three and a half and four diameters long respectively; and if using the best coal, they ought not to be more than five and a half and six diameters long.

Boilers whose dimensions are proportioned within the limits stated in the above practical rules, more especially those indicated by the three last, are mostly the best to be found of each kind in the manufacturing and midland counties.

The manner of hanging boilers, to which the first rule relates, is yet generally confined to some country places where inferior workmanship only can easily be obtained, and where the space occupied by the greater length of the boiler is not of much value; consequently the data for this rule have not perhaps been quite so exactly determined as may be. There are reasons for supposing that the direct draft cylinder boiler may be made considerably shorter than the proportion of six to one, as stated in Rule 1, if the fire could be equally well arranged as in the wagon boiler, the concave bottom of which is admirably adapted to this purpose, because the middle of the grate, where the heat is the most intense, is at the greatest distance below the boiler bottom, while the latter gradually approaches nearer to the grate at the sides of the furnace, and thus tends to equalize the action of the fire against the boiler. The bottom of the cylindrical boiler being convex downwards, the action of the fire is of course exactly the reverse of the above. To say nothing of the injury done to the boiler plates on this account, there requires to be a greater average distance between the grate and the boiler bottom; this, again, requires a greater quantity of coal in the furnace, which impedes the draft, and renders stoking necessary, thus causing the flame to be occasionally extended in length; and although such undue length of flame cannot be kept up continuously with any degree of economy, it has given rise to a natural, though unfounded, prejudice against this mode of setting a boiler, often expressed in the observation that "all the heat goes up the chimney." Erroneous as the idea is that gives rise to this very common remark, it is not a little strengthened by the fact that the temperature of the chimney is always very much greater with a direct draft than it is where winding brick flues are used, which may be considered only as a portion of the chimney lying horizontally, the superfluous heat in which is doing no good, and is really "waste heat," from the great inconvenience and trouble it occasions; whereas, when this waste heat is allowed to go freely

up the vertical shaft of the chimney, it really becomes of great use in increasing the ascensional force of the current; thus improving the draft, and enabling the boiler to be worked generally with the damper nearly closed, as all steam engine boilers ought to be worked.

Boilers on the Oven Plan Liable to Explosion from Surcharged Steam.*

When a boiler is set up or "hung" with a direct draft, as described in Rule 1, it is very commonly, but erroneously, said to be on the "oven plan," which designation ought to be confined to those only where the flame is caused to pass wholly or partially over the top of the boiler, although they have generally a direct draft also. The use of such plans, however, cannot be too much reprehended as pregnant with danger, from surcharging the steam with heat, and thereby becoming liable to explosion. Few, however, if any, are now set up in that way; but I cannot help thinking that some of the otherwise unaccounted for explosions which have occurred of late years have been owing to a similar cause.

Now, supposing this over-heating of the steam and the top of the boiler to occur from either of the above causes, while the engine is at work, it is not perhaps likely that anything particular will be observed to ensue, unless, probably, the burning of the clothing or other covering of the boiler. But let us examine the matter when the engine has been standing for some time, or when just about to start after the first getting up of the steam, and we shall find a very different state of things, the consequences of which, if only leading to the slightest probability of resulting in an explosion, are too serious to be passed over without great consideration.

We may suppose, then, that the steam chamber has become filled with over-heated or surcharged steam from any cause whatever, whilst the top of the boiler is also in the condition already described, that is, exposed to a temperature of, say from 350° to 400°; which supposition is quite consistent with the fact that the great bulk of the water in the boiler may be at the same time considerably below the common boiling point. And although this last assumed fact is, perhaps, only of rare occurrence in boilers which have their fires underneath them, it cannot be too widely known, that in all stationary boilers which have internal furnaces, it is not only of frequent occurrence, but it is a common observation of most practical engineers and workmen, that at the first getting up of the steam from cold water in a boiler of this description, with the safety valve and all other outlets of the boiler closed, the water at the lower part of the boiler is often quite cold, whilst there is a pressure in the boiler of 10 or 20 lbs. on the square inch.

As my object in thus tracing the proximate causes concerned in producing the dangerous consequences that may ensue, is for the purpose of suggesting the means of prevention, it will be here useful to observe, that whatever may be the pressure of the steam due to the actual evaporation at this stage of the process, it will be nearly doubled by the heat of the steam chamber, supposing the latter to have arrived at about 400°,

From the London Artizan, for January, 1851.

which is still under the temperature that would leave any permanent traces of its existence on the metal, the lowest temperature that gives the first discoloration to iron (a straw color) being about 430°. Now, excepting by the thermometer, there is only one other means of giving the fireman any ready indication or suspicion of there being any thing wrong about the boiler; and rough and uncertain as that indication is, it is always sufficient to create alarm, and induce him to take some instant precautionary step; and whether that step be properly or improperly taken, will make all the difference between hastening and preventing the catastrophe of blowing up the boiler. The indication I am alluding to may be thus described-whenever a fireman finds that, on getting up the steam previous to starting an engine, more particularly after a boiler has been at rest, and refilled with cold water, that the pressure rises to a certain height in about half the time that has usually been the case previously; or if, after firing a certain time, and consulting his steam gauge, he finds that the steam has run up to 10 or 20 lbs. pressure, instead of 5 or 10 lbs., as he expected, in the same time, and if the man has been accustomed to the same boiler, and is assured there are no unusual circumstances, of draft or otherwise, to account for the difference, then he may be almost certain that there is surcharged steam in the boiler.

Now, an idle or a careless stoker might not perhaps notice the circumstance just described, so long as the required pressure is not reached, which the safety valve is loaded to blow off at; which is the case no doubt very frequently, and the circumstance is thus passed over without danger, or the suspicion of danger. But what is the step that a careful, and still more, a timid, man would be likely to take at such a time? In all probability, if the means were at hand, he would either "feel" or lift the safety valve; and what is the consequence that would be most likely to follow? In the case we have supposed, it certainly would not be safety, but danger, and very possibly immediate destruction; for when a safety valve of any considerable area is thrown suddenly wide open, there is always a sudden rush of water, or rather water and steam mixed, to the opening. This is well known to occur universally under such circumstances, and I have more than once seen it purposely exhibited by foolhardy engineers, by way of illustrating the nature of priming, as priming in fact it is, at the safety valve instead of into the cylinder.

We need not, however, suppose that this is exactly the course taken in the case under consideration; for a very careful man would perhaps only ease the safety valve gently on its seat; and if it was in a cotton mill or other factory requiring to be heated by steam, the almost certain course he would take would be to let the steam into the pipes for that purpose; and whether he opens the communication suddenly or slowly, the effect produced is, that the surface of the water is simultaneously relieved from a portion of the pressure, and in consequence of being so relieved, the water immediately commences boiling, not so violently perhaps on the instant, but sufficiently so to change it from its hitherto quiescent condition to a state of active circulation at least, if not of actual ebullition at the surface. Now this, or any other disturbance of the surface of the water, starting the engine for instance, will rapidly supply the surcharged steam with its full complement of water, that being all that is wanted to cause

the pressure to mount up suddenly from 20 to 200 or 300 lbs., perhaps in a few seconds, or to such pressure as is due to the final temperature of the steam when fully saturated with water, the dry surcharge or desiccated steam, as it may be called, suddenly becoming ordinary steam of somewhat less temperature, but enormously increased in density and pressure, with what effect on the boiler of course depends entirely upon the strain it is capable of bearing without rupture.

Supposing the iron to have arrived at about 400° Fahr., full saturated steam in contact with it will assume a pressure of about 215 lbs. per square inch above the atmosphere, a pressure quite equal to account for many of the disastrous explosions we have on record.

For the Journal of the Franklin Institute.

Notes on the U. S. War Steamer "Spitfire." By Chief Engineer B. F. ISHERWOOD, U. S. N.

The "Spitfire" was originally built for the Mexican government, but at the commencement of our late war with Mexico she was still in the hands of the builders, from whom she was purchased by the United States and sent upon the Mexican Gulf coast, where she was exceedingly serviceable in expeditions against the interior; her small size and light draft of water allowing her to cross the bars and ascend the rivers, while her battery was sufficiently heavy to be very effective in the bombardment of the towns. Throughout the whole war she proved a very useful and efficient war steamer for the peculiar river and town warfare she was engaged in. A brief record of her dimensions, power, speed, &c., will be valuable, as it is only such class steamers that can be rendered truly available either for attack or defence on the Mexican Gulf coast, or the southern United States coast. Our large war steam frigates, the "Mississippi" and "Princeton," which were also on the coast of Mexico during the war, though invaluable for towing and dispatch vessels, were useless as war steamers from their great draft of water, viz: 19 feet, which prevented them from ever approaching the sandy shores of the Gulf; and as the Mexicans possessed no marine, our large ships of war were completely useless from the want of objects on which to use their batteries. HULL.-The "Spitfire" was 118 feet long on deck, 224 feet extreme beam, 9 feet depth of hold, and 241 tons burthen. Mean draft of water, 7 feet.

BATTERY.-One eight inch gun, (68-pounder,) mounted forward on a pivot, and two 32-pounder broadside guns aft.

ENGINE. One half beam engine (Lighthall's patent,) cylinder lying horizontally over the keelson. Diameter of cylinder 36 inches, stroke of piston 6 feet. The main cranks, however, were only 24 feet between centres, the connecting rod being attached nearer to the centre of motion of the beam than the piston rod. The engine had a piston valve 18 inches diameter, packed by rings set out with screws. This valve was of the length of the cylinder, and had no lap. The steam was worked expansively by a separate slide cut-off, situated on the opposite side of the cylinder, and cutting off at a little beyond half stroke. The capacity of