166 



THE CIVL ENGINEER AND ARCHITECT'S JOURNAL. 



[May, 



of the load, then, which the engine can draw, will depend chiefly on the 

 adhesion, and the velocity will depend on the boiler where the stream is 

 generated, the cylinders being proportioned to each of these two other 

 regulating powers. And not only must the steam be generated to a given 

 pressure to produce that power, but with sufficient rapidity to continue it ; 

 and in keeping up a high velocity, it must be, as it were, rammed into the 

 cylinders, so as to produce the greatest possible effect in the least possible 

 time, and this is the reason why high velocities are so very expensive, as the 

 same effect might be produced by one-fourth the quantity of steam, if suffi- 

 cient time were given to expand it. But there is yet another circumstance 

 that modifies the amoimt of adhesion — viz. the inclination of the roiid. It is 

 manifest that, if the road were vertical, the engine could have no adhesion 

 upon the rails; and, therefore, between the perpendicular and horizontal 

 lines, the power must undergo many degrees of variation, quite independent 

 of the atmospheric causes already mentioned. AVe have no experiments to 

 determine the ratio of that variation, but, reasoning from analogy, it may 

 be assumed to be as the sine of the angle of inclination, or in the same pro- 

 portion as the resistance arising from gravity, so that practically the 

 diminished amount of adhesion, on any inclined plane, might be found by 

 deducting the resistance of gravity on that plane from the constant of 1000 

 given above ; thus, on an inclination of 1 in 100. the gravity of the engine 

 per ton (or 2240 lb.) will be -.jSjif = 22'4 lb., and that for seven tons will be 

 22-4x7== 157 lb., which, subtracted from 1000, will give 843 lb. ^ the 

 diminished amount of adhesion, which will be the limit of the power of the 

 engine on that incline, as regards the load, no matter how great the boiler or 

 cylinder power may be. And to find the load which this power will draw, 

 we must take the sum of the resistances arising from gravity and friction for 

 one ton, and the adliesive power divided by this sum will be the amount 

 sought in tons; on an inclined plane of I in 100, the calculation will be found 

 thus: — Friction 9 1b. per ton, plus gravity as before, 224 =3r41b., and 

 adhesive power 843, divided by 31'4 = 267 tons, which is only one-fourth of 

 what might be drawn on a horizontal line. Hence the advantage of heavier 

 engines, which are daily coming into use, as also the propriety of coupling 

 the wheels of engines for drawing heavier loads up steep inclines, and I^y this 

 means the whole insistent weight of the engine is rendered etfective by ad- 

 hesion, and the load the engine can draw after it proportionally increased. 

 In calculating the amount of resistance of a load upon a railway, the friction 

 had been assumed to be 9 lb. per ton, rather in deference to general opinion 

 than otherwise; it w.as probably much higher. It is considered that the 

 friction of the engine and engine gear is 16 lb. to the ton, but that of the 

 lighter carriages less ; however, if this number (9 lb.) should be proved in- 

 correct by future experiments, the principle of the calcidation will not be 

 altered, and it will only be necessary to substitute the true number instead 

 of 9. The same may be said with regard to the adhesion ; it will only be 

 necessary to substitute for 1000 whatever number shall be found on closer 

 investigation to be nearer the truth. 



The power generated in the boiler, and applied in the cylinders, now re- 

 mains to be brought under consideration. This may be stated to be the ca- 

 pability of the boiler to supply steam of high pressure, to enable the piston 

 to perform a given number of stroke.=! per minute, which accordingly will be 

 one of the essential elements in computing the power cf the engine ; and 

 therefore it is that we are always unwilling to define it by any number of 

 horses' power, since it is clear that the engine which, moving at the rate of 

 15 miles an hour, would be called a 20-horse engine, would be styled a 40- 

 horse engine when moving at the rate of 30 miles an hour, all other circum- 

 stances remaining the same. But it does not follow that, because the num- 

 ber of strokes per minute be increased, that the power available for locomotion 

 be increased also, and in this consists the essential difl'erence between loco- 

 motive and stationary engines, for in the former there are circumstances, as 

 before shown, which circumscribe that power, over which the boiler has no 

 control ; and, as regards the locomotive engine, a third point must be taken 

 into consideration. It is a well-known theory, that, if a metallic substance 

 be in contact on one side with water, and that heat be applied to the other, 

 that once the body becomes thoroughly warmed, the caloric will be taken up 

 by the water with as much rapidity as it can be supplied to the metal. Now, 

 in the locomotive engine, there is an immense area of heating surface in con- 

 tact with the water in the boiler, in consequence of the numerous tubes which 

 pass through it from the fire-box to the chimney, and it is on this principle 

 that what is called the steam draft has been introduced, by which means the 

 caloric is rapidly drawn from the fire through these tubes, and as rapidly 

 absorbed by the water with which they are in contact, for the production of 

 steam. It is evident that, in proportion to the rapidity with which the piston 

 moves, and with which the waste steam is injected into the chimney, will the 

 heat be absorbed by the water from the tubes and steam generated, the efiect 

 of which is, that the faster the engine goes, the quicker it generates the 

 steam ; and this forms another great beauty and peculiarity in the loco- 

 motive engine. The principles of calculating the moving power being thus 

 explained, the way has been sufficiently cleared for entering on the subject 

 of the laying out of railways. 



Lecture III.— After recapitulating a few of the leading points which were 

 stated in the last lecture, the Professor called particular attention to the 

 formula whereon he had based the calculations into which he had then en- 

 tered, and he now exhibited tables and diagrams in further illustration. 

 Tlie adhesive power of 1000 lbs. was assumed as the average of what a loco- 

 motive engine will have in all states of the weather, and of the rails ; but if 

 the wheels be coupled, or the insistent weight otherwise increased or di- 

 minished, the adhesive power ("on which depends the load) will be altered in 

 the same proportion, subject also to variation from the state of the weather 

 and the road, and undergoing the stated diminutions from the eHi>cts of 

 gravity on all planes which depart from a horizontal line, the velocity of 

 the train depending on the evaporating power of the boiler. But m the 

 stationary system the engine winds (upon a roller, or over a sheave or wheel) 

 a rojie supported by puUies, placed at regular distances along the road, and 

 to which rope the train is attached. Mr. Vignoles stated that the student 

 may refer with confidence for every information on this subject, to Mr. 

 Wood's Treatise on Railways, and commented on the extracts he made from 

 that work. 



Atmospheric Railway. — There is also another mode of applying the station- 

 ary engine to the purposes of locomotion, by producing through an air pump 

 a partial vacuum in a pipe, thus making atmospheric pressure the moving 

 power ; and it may be interesting to state, that the scientific men who were 

 appointed by the railway department of the Board of Trade to inquire into 

 the .system of the Atmospheric Railway, had fully recognised that principle, 

 and concurred in considering that the experiment contemplated upon the 

 Dublin and Kingstow n Railway extension, and recommended by the directors 

 to the proprietors, as applicable for illustrating the principle on a large scale. 

 On the atmospheric railway the diameter of the pipe or tube regulates the 

 load, but the velocity depends almost entirely upon the diameter of the air 

 pump that exhausts the pipe, the rule being that the area of the air pump 

 must be made as many times greater than the area of the pipe, as the velocity 

 of the train is to exceed that of the piston of the air pump. Thus, if the 

 piston of the air pump be supposed to move at a rate of three miles an hour, 

 and it be required to move a train at a velocity of thirty miles an hour, the 

 area of the air pump must be made ten times the area of the pipe ; the 

 diameter will, of course, be deduced from that area. Now, it appears that 

 the most economical pressure in the pipe (which is what engineers must 

 chiefly look to.) is about 7 lb. to the square inch, or rather less than half a 

 vacuum ; therefore, this may be taken as the constant of the atmospheric 

 pressure ; and if we multiply this constant by the area of the travelling piston 

 in inches, we shall obtain the efl'ective pressure upon that piston, which, as 

 it regulates the load, may be said to correspond to the adhesive power in the 

 locomotive engine, but which, unlike that power in the locomotive, will be 

 undiminished on inclined planes. Again, if we divide this power by the 

 friction (which was before taken at 9 lb. to the ton), we shall obtain the 

 number of tons which the piston, acted on by the atmospheric pressure, is 

 capable of propelling. Thus, supposing we have a pipe of 14 inches diameter, 

 if we multiply the area of this pipe by 7 lb., we shall find the effective pres- 

 sure equal to 1078 lb., which, divided by the friction, 91b., will give about 120 

 tons — the weight which can be propelled by means of a pipe of that diameter ; 

 and if the piston of the air pump move at the rate of three miles an hour, 

 and Its area equal to seven times that of the pipe, the load will be moved 

 with a velocity of twenty-one miles an hour, and it may be demonstrated 

 that, on ascending anil descending planes, the speed, although increased or 

 diminished at first, will soon become uniform. Of course, upon the diameter 

 of the air pump will depend the power of the engine which is to work it. The 

 calculations in this case will be similar to those for an engine required to 

 work ropes — in the one case it being required to find what is wanted lo over- 

 come the resistance and friction from ropes, pullies, &c., and in the l.^tter to 

 find the power to work the air pump, and exhaust the air from the tube at 

 any required velocity. 



Inclined Planes, — The Professor then recurred to the eftect of trains descend- 

 ing inchned planes. Mr. Navier (in his work, translated by Mr. M'Neill, 

 which he mentioned as a text book on the comparison of different lines of rail- 

 way) differed somewhat from the propositions he had laid down: it was therein 

 stated, and Professor Barlow concurred in the statement, that an engine and 

 train did not gain any advantage in descending planes steeper than a certain 

 inclination which they have put as the angle of repose. Now, in practice, 

 Mr. Vignoles did not find it so, but, on the contrary, daily experience proved 

 that, as far as inclinations of sixty feet in a mile, the trains may, under 

 almost all circumstances, have the full benefit of gravity in the descent. 

 Professor Barlow has laid down, in several important works, which from 

 their high standing, will have a material influence upon the public mind, that 

 though additional power be required to surmount steep inclinations, yet, so 

 far from gaining a corresponding advantage in the descent, there will result 

 rather an injurious efiect from the necessity of applying the break. Now, 

 it has been already mentioned, and experiments have been repeatedly made 

 by Mr. Wood, Dr. Lardner, and others, showing that, when engines descend 

 long inclined planes, such as those on the Croydon Railway, the application 



