June 3, 1897] 



NA TURE 



'03 



isistance on the engine shaft, independent of the speed, 

 11 the work done being converted into heat, which 

 ippears in the rise of temperature of a steady stream of 

 irater flowing through the brake, the magnitude of which 

 stream is independent of the load on the brake or the 

 speed of the engine, and is under independent control. 



The existence of the Manchester town's water, of a 

 )urity expressed by 3 grains of salts to the gallon, 

 :onveniently distributed in the laboratory, as well as 

 luxiliary power, both steam and water. 

 Although unconsciously, the research was really com- 

 S^menced in 1890, when, without any intention of making 

 ^■a determination of the heat equivalent of the work done 

 on the brake, but solely for the purpose of verifying the 

 mechanical balance of the brake, provision for thermal 

 measurements was added, and a system of trial instituted, 

 in which the object sought was only that of obtaining 

 consistent results over definite portions of the scales of 

 uncorrected thermometers, eliminating the errors re- 

 sulting from radiation by taking the differences of two 

 trials. In these trials the temperature ranged from 40° 

 to 50^ F., and their development was continued over two 

 years. Then it occurred to Prof Reynolds that by the 

 same method the great facility which this brake was then 

 seen to afford would be available for the independent 

 determination of the mechanical equivalent, if it could be 

 arranged that water should enter the brake at the 

 temperature of melting ice, and leave it at that of water 

 boiling under the standard pressure. Since then all that 

 would be required of the thermometers would be the 

 identification of these temperatures, and with a range of 

 180^ the error in scale over one or two degrees would 

 be comparatively of small importance. At first the 

 difficulties seemed formidable ; but on trying by gradu- 

 ally diminishing the supply of water to the brake when 

 it was absorbing 60 H.P., and finding that it ran steadily 

 under control of its automatic gear till the temperature 

 was within three or four degrees of boiling, he further 

 considered the matter, and during the next two years 

 convinced himself of the practicability of the necessary 

 additional appliances by preliminary designs. These 

 consisted of: — 



(i) An artificial atmosphere or means of maintaining a 

 steady air pressure of 4/3 atmosphere in the air passages 

 of the brake. 



(2) A circulating pump and water cooler, by which the 

 entering water, some 30 lbs. a minute, could be forced 

 through the cooler into the brake at a temperature of 

 32^, having been cooled by ice from the temperature of 

 the town's main. 



(3) A condenser by which the water leaving the brake 

 at 212' might be cooled down to atmospheric temperature 

 before being discharged into the atmosphere and weighed. 



(4) Such alteration in the manner of supporting the 

 brake on the shaft as would prevent excess of leakage 

 from the bushes in consequence of the greater pressure 

 of air in the brake, since not only would the leaks be 

 increased, but when the rise of temperature was increased 

 to 180' the quantity for any power would be diminished 

 to one-sixth of that for a rise of 30^, so that any leakage 

 would become six times the relative importance. 



(5) Some means by which assurance of the elimination 

 of the radiation and conduction could be obtained, as 

 with a temperature of 140^ F. above the laboratory these 

 would probably amount to 2 or 3 per cent, of the total 

 heat. 



(6) Scales for greater facility and accuracy in weighing 

 the water with a switch actuated by the counter. 



(7) A pressure gauge or barometer by which the 

 standard for the boiling-points might be readily deter- 

 mined at 3^ or 4" F. above and below the boiling-point, 

 so as to admit of ready and frequent correction of the 

 thermometers used for identifying the temperature of the 

 effluent water. 



NO. 1440, VOL. 56] 



(8) Some means of determining the terminal differ 

 ences of temperature and quantities of water in the brake, 

 which would be relatively six times as large as with 30' 

 rise. 



These preliminary designs apparently demonstrated 

 the practicability of the appliances, and also the possi- 

 bility of their inclusion in the already much occupied 

 space adjacent to the brake. But there remained still 

 much to be done in the way of experimental investiga- 

 tion in order to obtain the data for proportioning the 

 appliances. 



In July 1B94, Mr. Moorby having undertaken to devote 

 himself to the research, the experiments necessary for the 

 appliances were at once commenced, and these, together 

 with the construction of the appliances and then standard- 

 ising, and preliminary experiments while this was in 

 progress, occupied till February 1896, when Mr. Moorby 

 commenced the main experiments, which were continued 

 into July 1896. 



In these experiments the time of running was 62 

 minutes ; the speed 300 revolutions a minute, on the 

 speed gauges. Observations of speeds, of- the tempera- 

 ture of the inflowing and effluent water, and of the tem- 

 perature of the air were made every two minutes. 

 Observations of the slope of the temperature of the shaft 

 were made every eight minutes. 



The temperature of the inflowing water varied from 

 32-5° to 34^, and that of the effluent from 210'' to 214^ F. 

 The effluent water was cooled to 8'5° before entering the 

 tank on the scales, in which it was weighed, weighings 

 being taken before and after each trial. 



The temperature of the metal surface of the brake 

 being sensibly the same as that of the effluent water 

 (212"), and by taking the difference in the work absorbed 

 in two trials and the differences in the heats developed 

 the errors of radiation and balance in the brake were 

 approximately eliminated, and in order to complete the 

 elimination the coefficient of radiation was approximately 

 determined, so that a correction might be applied for any 

 residual differences of temperature as observed, and in 

 the same way with the slope of temperatures. Further 

 assurance was also obtained by making some trials with 

 the brake naked and others with it covered, so as to 

 reduce the loss of heat to one-fifth ; and, in the same 

 way, with every circumstance which could effect the 

 result trials, means were taken to vary the circumstances 

 in different series of trials, so as to obtain an estimate of 

 the limits of error. All the appliances were most care- 

 fully standardised ; and taking all the limits of error 

 into account the limit of the sum was less than 0*0003. 



In all fifty-two trials were included in the final result. 

 Of these— 



25 with loads on the brake of 1200 ft.-lbs. at 70 H.P. 

 21 ,, ,, 600 ,, 35 ,, 



6 ,, „ 400 ,, 23 „ 



From these, twenty-five separate determinations were 

 made of the equivalent, subject to certain general cor- 

 rections given below, which gave a mean value 777 "91, 

 and from this none of the separate determinations dif- 

 fered by as much as 0*2 per cent., which arranged in 

 eight groups, according to the circumstances under which 

 they were made, the greatest divergence from the mean 

 was o'o5 per cent. 



It was found impracticable to eliminate entirely from 

 each determination the losses of heat due to radiation, 

 conduction, leakage of water, &c., and so it was found 

 advisable to determine what these losses were. This 

 information was given by the trials themselves, and the 

 necessary corrections were applied to each separate de- 

 termination. As illustrating the extent to which the 

 method adopted did eliminate these errors, it is interesting 

 to remark that on the mean value of the equivalent de- 

 termined, without taking these errors into account, the 

 error was only o"oi92 per cent. 



