Sept. 19, 1889] 



NATURE 



515 



world — it is unit of weight multiplied by unit of height — and 

 with us the foot-pound, or 33,000 limes the foot-pjund, is gene- 

 rally accepted ; but the difficulty lies in determining how the 

 measure is to be applied. Thus, in the case of a water-motor, 

 should the power be calculated by the energy latent in the falling 

 water, or in the actual work given off by the motor ? In heat- 

 engines we have to deal with many variables. There is the 

 initial pressure of the working agent, the terminal pressure, the 

 length of stroke, the number of revolutions per minute, the 

 indicated pow er in the cylinder, the effective power given off, 

 and the adequacy of the means of supplying the working agent. 

 In the early days of steam, when pressures were pretty uniform, 

 and speed bore a certain relation to the stroke, the diameter of 

 a cylinder was a tolerably close index to the ]iower of the 

 engine, and such simple rules as "10 circular inches to the 

 horse-power," which prevailed among agricultural engineers, 

 were tolerably intelligible. But in these days, when pressures, 

 speeds, and rates of expansion vary so greatly, the size of the 

 cylinder, or cylinders, is no longer a guide, and I imagine that 

 most manufacturers have ceased to class their engines by their 

 nominal horse-power. The problem is pretty simple in the case 

 of pumping-engines, for there the nominal power may be taken, 

 as it is in Holland, to be the actual work performed upon the 

 water, and perhaps a similar rule might apply to motors driving 

 dynamos, but for most other purposes no simple law is possible. 

 In my own practice I have, for many years, lieen in the habit of 

 classing engines by their indicated horse-i)ower per cne revolu- 

 tion for every probable initial pressure, below the maximum one 

 for which the engines were designed, and for various rates of 

 expansion. To facilitate the calculations I use curves which 

 give the theoretical horse-power, on the supposition that steam 

 expands according to Boyle's law, for 10,000 cubic inches of 

 steam measured at the moment of exhaust, which is, in fact, the 

 volume of the cylinder in single-cylinder engines, and the volume 

 of the last cylinder in compounds. These curves are calculated 

 for initial pressures rising by 25 pounds, and, in non-condensing 

 engines, for the extreme range of expansion po-sible, and to 

 fourteen expansions in condensing engines. The true indicated 

 horse-power ranges from 80 per cent, to 85 per cent, of the 

 theoretical, as above stated, the preci-e percentage depend- 

 ing upon the construction of the engine. As large engines 

 are now almost always compound, the size of the cylinders 

 is no guide to the lay mind ; hence, in answering in- 

 quiries, it is necessary by some means to get at the actual 

 horse-power expected and to settle the initial pressure, for on 

 this point there is still much timidity among steam-users, so that 

 the engine-builder has to adapt himself in this and other 

 particulars to the needs or prejudices of his customer. 



In marine engines, again, the difficulty is still greater, because 

 the only measure of the effective power of the engines is the 

 speed of the ship under given conditions of immersion. But the 

 resistance of ships is a complicated matter, not perfectly ascer- 

 tained yet, so that the speed attained in any new combination of 

 engines and hull is by no means a certainty ; hence some recog- 

 nized measure of the power of a marine engine, depending only 

 on the measurement of the cylinders and boilers, becomes very 

 desirable. 



So strongly has the want of a standard horse-power been felt 

 by shipbuilders and marine engine makers, that the Council of 

 the North-East Coast Institution of Engineers and Shipbuilders 

 appointed a Committee to investigate the subject, and to devise, 

 if possible, a set of rules which would be generally acceptable. 

 The Committee made its report in the spring of 1888. They 

 took as their basis the indicated horse-power, under certain 

 normal conditions, and propose to call this the normal indicated 

 horse-power (N.I. U. P.). The normal conditions are, briefly, 

 the following : — 



(i) That the steam, of whatever boiler pressure, is expanded 

 to the same terminal pressure. 



(2) That the expansion is effected by all engines with the same 

 degree of efficiency. 



(3) That the piston speeds of engines of different lengths of 

 stroke are proportional to the cube root of their respective 

 strokes, and, further, that the actual loaded trial-trip value of 

 piston-speed may be taken as 144 times the cube root of the 

 stroke in inches (i44\''s")- 



(4) That in cases in which the engines and boilers bear to 

 each other such proportions as to prevent condition (i) from being 

 fulfilled without thereby violating condiii^n (3) the coal consump- 



tion per I.H.r. will not be affected, but will be constant for the 

 same boiler-pressure. 



(5) That the boilers are constructed in accordance with what 



will be generally recognized as the average practice of the 



present day in respect of the allowance of steam room in 



relation to power, the diameter, area, and pitch of tubes, the 



relation of grate to heating surface, and the area of uptakes 



and funnel ; that average natural chimney draught is used, or, if 



forced draught be employed, that it does not exceed the natural 



draught ; that the horse-power is proportional to the heating 



surface (H), and to the cube root of the pressure (^/P) ; and. 



further, that the actual loaded trial-trip horse-power may be 



taken as equal to one-sixteenth of the heating surface multiplied 



(H ^/P) 

 by the cube root of the pressure ■ — %-- - 



(6) That the efficiency of the engine mechanism is constant, 

 and that the propeller is such as to secure that the engines will 

 utilize the boiler power referred to in condition (5) in the 

 manner prescribed by conditions (3) and (4). 



Subject to these conditions the normal indicated horse-power 

 is found by multiplying the square of the diameter of the low- 

 pressure cylinder in inches by the cube root of the stroke in 

 inches, adding to the product three times the heating surface 

 of the boiler in square feet, multiplying the sum by the cube 

 root of the pressure, and dividing the product by 100. 



N.I.H.P. 



(p2 4/S + 3H)^P^ 

 100 



It is evident from this formula, and from the conditions, that 

 account is taken of all the variables, and that the boiler is 

 regarded as an integral part of the engine. The report gives 

 several useful formula: deduced from the above. Whether the 

 expressions given are the most convenient possible for general 

 marine practice or not, I am not competent to say ; but it seems 

 to me that a step has been taken in the right direction in the 

 attempt which has been made to measure marine engines by 

 some rational standard. The members of the Committee were 

 all thoroughly practical as well as scientific men ; they deter- 

 mined their constants by reference to a large number of success- 

 ful cases ; and I sincerely hope that the question will be pursued 

 by the marine engine builders on the west coast, and by the 

 constructors of land engines. As engineer to the Royal Agri- 

 cuhural Society, I have frequently had to define the power of 

 engines entered for competition for the Society's prizes, and I 

 have experienced the greatest difficulty in laying down rules for 

 the guidance of intending competitors, being fearful, on the one 

 hand, of restricting originality, and, on the other, of admitting 

 engines of greatly varying powers. 



I have expressed an opinion that the numerous engineering 

 Societies which exist at this day have it in their power to pro- 

 mote the advancement of mechanical science in a notable manner 

 by appointing Research Committees, or by aiding individual 

 investigations from their abundant means. The North-East 

 Coast Institution of Engineers and Shipbuilders has done good 

 service in their endeavours to establish a practical measure of 

 the power of marine engines, while the Institution of Mechanical 

 Engineers has, for the last ten years, been steadily promoting 

 researches of an eminently practical nature. Their expenditure 

 has reached the handsome sum of ,^1700, and their Proceed- 

 ings have been enriched with Reports on the hardening, temper- 

 ing, and annealing of steel, on the form of riveted joints, on 

 friction at high velocities, on marine-engine trials, and on the 

 value of the steam jacket. The names of those who are acting 

 on these Committees are a guarantee that the investigations 

 conducted by them will rank among the classical works of the 

 profession, and will abundantly justify the liberal expenditure 

 which has been incurred. 



It is impossible to conclude the address which I have had 

 the honour of delivering, without an allusion to the most im- 

 portant structure which engineering skill and enterprise has ever 

 attempted. The P'orth Bridge is rapidly approaching comple- 

 tion, and on Saturday, September 14, Mr. Baker is to deliver 

 a lecture, in which he will, no doubt, tell us when the great 

 work is likely to be completed. I think that the members of 

 this Section belong sufficiently to the "working classes" to have 

 a claim to admission to the lecture, and to hear from the lips of 

 the creator of the bridge the story of its inception, of its progress, 

 and his hopes as to its completion. 



