226 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



July, 



NOTES ON STEAM NAVIGATION. 



Bo:leks. — II the steam space be contracted, there will inevitably be a 

 large coiisiimptioii of fuel as well as a low degree of efficiency in the engine; 

 for the iteam will be mixed with particles of water, wliich will carry oft' 

 much heat, and this water by entering the cyhnder will endanger tlie safety 

 of the engine, at the same time that it diminishes its power by resisting the 

 motion of the piston, blocking up the ports and choking the condense)-. It 

 is o;ily when the quantity of water which comes over with the steam is very 

 great that these eft'ects are obvious ; but eveiy particle of water mingled with 

 the steam is a positive detriment, and the only effectual preventative of the 

 evil is to make the steam space abundantly large. Mr. Watt's proportion, 

 which is well known, is we believe better than any other. 



It is a usual and judicious practice to take tlie steam from the highest part 

 of the boiler or steam chest, generally by an internal pipe turning upwards 

 and in continuation of the steam pipe. This internal pipe is sometimes made 

 of malleable iron, in which case it rapidly oxidizes, and in the course of a year 

 ortwo will often disappear altogether. Copper is the best material, and around 

 pipe is preferable to a square or flat one. The latter description we have 

 known to have had its sides squeezed together by the difference of the 

 pressure of the steam witliin and without it. 



The oxidation of boilers is an inexplicable subject. The outsides of boilers 

 are chiefly worn — on the bottoms by leakage, or the action of the bilge 

 water — in tlie ashpits from the wetting of the ashes — and on the top from 

 the dripping of water fiom the deck. The insides are subject to oxidation 

 only above the water line — not equally over the whole surface of the iron 

 bounding the steam space — but more in one part than another, and the differ- 

 ence is traceable to no specific cause with which we are acquainted. It is 

 ge:ierally observable, however, that oxidation proceeds most rapidly upon 

 those parts upon which the steam impinges, and also upon the parts adjacent 

 to brass or copper. The iron in the neighbourhood of the safety and stop 

 valves is generally the most rapidly corroded ; but this eflfect is not invariable, 

 nor is it known to what the want of uniformity is due. The oxide is some- 

 times detached in a number of consecutive black scales, like the leaves of a 

 book, which fall oft' only when disturbed; in other cases no oxide adheres to 

 the surface of the iron, and the only evidence of the existence of oxidation 

 is the diminution of thickness in the plate, which is readily observable where 

 two plates overlap. A very thin coat of Roman cement might be applicable 

 to the interiors of steam chests with advantage, either in conjunction with 

 pieces of zinc, or zincked iron, or pe>- ne. 



/{anting bridges. — Every boiler should have a hanging bridge at the ex- 

 tremity of the flue where it enters the chimney, projecting downwards 18 in. 

 or 2 ft. from the top of the flue, according to the depth of the flue and other 

 circumstances ; and the best boilers are so provided. This hanging bridge 

 toay be made either of brick or of plate iron ; its efl'ect is to retain the hot 

 air in contact with the iron, and to permit that portion only which occupies 

 the lower part of the flue, and is consequently the least hot, to escape to the 

 chimney. A hanging bridge at every turn of the flue has in some instances 

 been used with much advantage. 



Furnace bars. — Where the length exceeds 7 ft. it has been found that 

 more steam is generated and less coal burned, by shortening them. They 

 must never be bound at both ends, nor the bearers be so constructed as to 

 permit ashes to accumulate in the space left to allow of the expansion of the 

 bars, else the expansion will either break the bearing bars or the brackets 

 which support them, or the middle portions of the bars will bend up into 

 the tire, and be quickly burned away. If there be two sets of bars in each 

 furnace the middle bearer should be double, to allow the ashes to fall into 

 the ash-pit through the space between the ends of the respective sets of bars. 

 The front bearer should also be removed a little from the dead plate, and the 

 back bearer a little from the bridge, for the same reason . 



Paddle Wheels. — The loss of beneficial eft'ect from backwater is not 

 great, but there is another source of loss hitherto unnoticed, though of no 

 trivial moment. The impulsive velocity of the several floats immerged in 

 the water is proportional to their horizontal distances, that is, supposing that 

 there are four floats immerged, the distance through which the first float 

 tends to impel the vessel during its motion from the position of the first to 

 th.at of the second float is equal to the horizontal distance between those 

 floats, whilst distance through which it tends to impel the vessel during its 

 motion from the position of the second to that of the third float being, in 

 like manner, equal to the horizontal distance between those floats, is a very 

 different quantity from the first. Thus there are two forces acting with 

 different velocities at the same time, and these forces must be to a certain 



extent antagonistic. If we suppose the vessel to be moving with the velocity 

 due to the motion of the float from the first to the second position, then the 

 excess of velocity due to the motion of the float from the second to the 

 third position is thrown away in giving a useless velocity to the water ; 

 whilst, if we suppose the vessel moving at the latter velocity, or at a velocity 

 approaching thereto, the floats entering the water become an actual impedi- 

 ment to the vessel's progress, and will carry a mass of broken water before 

 them, in the same way as the bow of the ship, and in practice we have seen 

 this occur. To prevent the loss of power from this source, the relative 

 velocities of the floats should be such that the horizontal distances travelled 

 over in equal times should be equal. 



Slide Valves. — It is always best for the faces on the cylinder to be 

 formed of face plates of cast iron, from IJ to 2 in. thick. When the engine 

 is of sufficient magnitude to render an expansion joint in the casing expe- 

 dient, the best mode of fixing the face plates is to screw them to the cylinder 

 with countersunk bolts, both of the surfaces having been jireviously planed, 

 and a little red lead interposed. In other cases it is as good a plan to fix the 

 face plates to the valve casing, the surfaces being planed as before. The 

 bolts which attach the casing to the cylinder will also serve to keep the face 

 plates in contact with the casing ; but they must be screwed at both ends, 

 and have a conical enlargement in the middle to fit into the holes of the 

 face plates, which holes must of course be made conical likewise. 



No large cylinder should be faced lying on its back, for its own weight 

 alters its form, and makes the valve face untrue; emery should not be used 

 in grinding a valve to its surface. The most approved practice is to make 

 the two surfaces first, as true as possible by planing ; next, to try the valve 

 on with a little reddening, and help the surfaces cautiously with a file where 

 required ; and finally, to polish with powdered Turkey stone and oil. 



The effect due to a given quantity of lap or cover upon the steam and 

 exhaustion sides of slide valves, as well as of that due to various degrees of 

 travel in the valve, are questions looked upon as very mysterious, and they 

 are certainly important. 



If .? = half the length of the stroke of the piston in inches ; 



s' = half tlie length of the stroke of the valve in inches ; 



/ = that part of the stroke completed before the steam is cut off in 

 inches ; 



^' = that part of the stroke completed before the exhausting port is shut; 



^" = that part completed before the exhausting port at the other end is 

 opened ; 



e= cover on steam side; 



c' = cover on eduction side ; 



Then < = s j 1 + cos. (2 sin. "') \ 



f = s< 1 +C0S. (2 sin. 



-1. 



1 + COS. (2 sin. 



-/)} 



(1) 



(2) 



(3) 



By these formulas the action proper to valves with any degree of cover or 

 throw may easily be computed. 



Cocks. — Every cock about an engine — gauge cocks and all — should have 

 a bottom and a stuffing box, else they will be very troublesome as soon as 

 they begin to leak ; blow-oft' cocks in particular are a perpetual annoyance 

 without this provision. The valves invented by Mr. Kingston of Woolwich 

 as substitutes for cocks are very ingenious, and we understand operate most 

 satisfactorily. All vessels should have these valves, or some analogous con- 

 trivance, at the place where the blow-off and injection pipes penetrate the 

 vessel's sides, both for the purpose of preventing the escape of water into 

 the ship in the event of the pipes being broken, and to permit the blow-off 

 and injection cocks to he re-gronnd, without involving the necessity of taking 

 the vessel into dock. 



NOTES ON THE CONSTRUCTION OF BillDGES OF CAST IRON, 



WITH SOME EXPERIMENTS ON THE STRENGTH OF CAST 



IRON GIRDERS OF LARGE DIMENSIONS. 



The basis of calculation of the strength of cast iron beams is derived 

 from the fact of a bar one inch square and 1 2 inches long breaking with one ton ; 

 consequently the square of the depth, multiplied by the thickness and divided 

 by the length in feet, gives the breaking weight in tons. In a table given in 



