1S43.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



77 



•whole difference in the height was occasioned by expansion; it became, 

 however, requisite to ascertain if such was positively the case, ami by 

 leaving the load on the bridge, and taking an observation again when 

 the metal was cool, we found that the arches had returned to their 

 original position. There is nothing surprising in this circumstance, 

 although without reflection it would appear rather paradoxical, and I 

 mention it, not with a view of exemplifying the effect of expansion, 

 which is of course understood, but because I consider the peculiar 

 system on which this bridge is constructed, to be favourable to the free 

 action of expansion and contraction, without thereby being subjected 

 to any strain at all detrimental to its general strength. 



The part of the bridge, most immediately exposed to the action of 

 heat and cold, and more particularly to the action of the sun, is the 

 arc itself, the upper part of the spandrils being protected by the pro- 

 jection of the platform, consequently large castings would be subject 

 to unequal contraction and expansion and consequent strain, which 

 must very much weaken them, whereas, in bridges constructed upon 

 this principle, the flexibility of the whole system is so great, that the 

 expansion of the tubular rib, by increasing the versed sine of the arc, 

 can lift the entire arch without subjecting any part of it to such a re- 

 sistance as would tend to diminish its strength. 



H. H. Edwards. 



London, Feb. 20, 1S43. 



VARIABLE ORIFICE OF THE BlAST PIPE OF LOCOMO- 

 TIVE ENGINES. 



In the February number of the Journal, I described a " Self-acting 

 Expansion Slide Valve" and in the course of the explanation, re- 

 ferred to having obtained patents abroad for an apparatus, by means 

 of which, the orifice of the blast pipe of locomotives, can be regu- 

 lated by the engine driver ; I will endeavour in this paper to describe 

 the apparatus, and to point out its general utility. 



The determination of the area of the orifice of the blast pipe, is 

 of importance in the construction of locomotives ; upon its proportion 

 depends the supply of a sufficient quantity of steam for the service of 

 the engine, and also its comparative effective pressure upon the 

 piston. It may be made so large, or so small, as to prevent the engine 

 from performing her allotted amount of duty ; and the application of 

 this blast of steam, may be considered (next only to the boiler itself), 

 the most useful invention in this beautiful machine, so essential a 

 complement thereto, that the locomotive engine would have been very 

 imperfect without it. The possibility of its successful application 

 having been ascertained, experience promptly indicated the extreme 

 limits of the area of the blast, within which the engine could exert 

 her power; but it still remains to be decided as an invariable rule, 

 what the exact size should be within these limits, to produce the most 

 useful effect ; and you will very seldom find any two engineers who 

 adopt the same sized blast, for engines of the same power. 



When the diameter of the orifice of the blast pipe is too great, the 

 energy of the blast will decrease, and the draught through the fire 

 will not suffice to generate the quantity of steam required to keep up 

 the speed of the engine; when, on the contrary, the diameter is too 

 small, the resistance behind the piston will become so great, in con- 

 sequence of the steam not being able to escape through the con- 

 tracted passage, as sensibly to reduce its effective power on the 

 piston. Within these two extreme limits (if an invariable orifice of 

 blast is adopted) it at first sight appears, that there must be an inter- 

 mediate point at which, if it could be attained, an engine would per- 

 form the greatest quantity of work, with the smallest quantity of 

 fuel. This intermediate point, if it can be admitted to exist, is ex- 

 ceedingly difficult to discover, because a locomotive engine has to 

 overcome a degree of resistance that is constantly changing, either on 

 account of the load, the action of the wind, the state of the rails, or 

 other causes. 



In the preceding paragraph, speaking of the extreme limits of the 



size of blast, within which an engine may work equally well, some 

 doubt is expressed as to whether there exists an intermediate point 

 that might be preferable, as enabling the engine to perform more ef- 

 fective duty. It is probable tint within a certain limit, the blast (if 

 iuvariable) may be made of any intermediate size, witliout sensibly in- 

 fluencing the average effect produced, the inconvenience and advan- 

 tage resulting from the change being so nicely balanced, that no sen- 

 sible difference could be discovered. If such is allowed t» be the 

 case, the efficacy of the variable blast must be manifest without a 

 trial. 



In order to diminish the resistance behind the piston at the return 

 of the stroke, the elasticity of the steam his been taken advantage 

 of; a chamber placed at the foot of the blast pipe, by allowing the 

 steam to expand on its escape from the cylinder, relieves the engine, 

 and has permitted the adoption of the most contracted orifice of 

 blast, that I have yet seen successfully employed. The greatest relief, 

 however, has been obtained by throwing off the steam considerably 

 before the piston arrives at the end of the stroke, thereby enabling it 

 to expand before the return of the piston, and thus very effectually 

 diminishing its resistance ; and although by so doing a portion of the 

 effective power of the steam is lost, it is at the same time a judicious 

 choice between two evils, and if not adopted, the discharge of the 

 steam from the cylinder at the moment of the return of the piston 

 would determine a powerful resistance to its free action, and so re- 

 duce the effective power of the engine. 



The contraction of the blast pipe being an inconvenience inse- 

 parable from the condition of generating a good supply of steam in 

 the locomotive boiler, it becomes important to partially remove this 

 inconvenience when practicable ; and as the state of the fire, and the 

 quantity of steam required, are frequently varying, it may be posi- 

 tively assumed that an invariable contraction of the blast pipe is an 

 imperfection, and that even supposing the question as to the best pos- 

 sible dimension were decided, and a generally admitted rule laid 

 down, it would only then be really perfect for some particular load, 

 and state of the fire ; and that with a different load, or state of the 

 fire, the determined orifice of the blast would be objectionable ; 

 therefore, it may be concluded that the faculty of regulating the con- 

 traction of the blast, so as to have full power over the production of 

 steam, must be in itself a desirable condition, and that if by the same 

 means, the average resistance behind the piston can be diminished, 

 the " ensemble " may be admitted to be a material improvement. 



It frequently occurs that there is either too much, or not enough 

 steam in the boiler; when there is too much, it is the usual custom to 

 open partially, and sometimes entirely, the fire door, so that by ad- 

 mitting a current of cold air into the fire box, and through the tubes 

 the production of steam will be diminished ; but this remedy is very 

 objectionable, and should be applied as seldom as possible, because 

 the rush of cold air may give rise immediately, or subsequently, to 

 leakage in the hoops, and must very much contribute to the destruc- 

 tion of the tubes, and to injure the boiler itself; whereas, if it were 

 possible to enlarge the orifice of the blast pipe on such occasions, the 

 fire would be damped, and the steam would fall, probably without 

 having to open the fire door at all. 



When there is not enough steam, the draught through the fire, in 

 consequence of the low pressure of the steam, and the slow motion of 

 the engine, will necessarily be less energetic than it ought to be, the 

 means of exciting the fire becoming inefficient at the time when its 

 assistance is most wanted. A good engineer will certainly tal. 

 that this occurs as seldom as possible, but there are accidental 

 over which he has not sufficient controul, and on such occasions the 

 power of contracting the orifice of the blast pipe would be \ery 

 beneficial, by enabling him materially to increase the rapidity with 

 which the fire would be brought up to its proper state. 



When a heavy train is going up a steep incline, its speed will de- 

 crease; the strokes of the engine being less frequent, the draught 

 through the fire will also diminish in intensity, and the quantity of 

 steam generated may no longer be sufficient; a slight contraction of 

 the orifice of the blast pipe would then increase the power of the 



