396 



NA TURE 



[August 24, 1882 



The greater efficiency of gas as a fuel results chiefly from the 

 circumstance that a pound of gas yields in combustion 22,000 

 heat units, or exactly double the heat produced in the combustion 

 of a pound of ordinary coal. This extra heating power is due 

 partly to the freedom of the gas from earthy constituents, but 

 chiefly to the heat imparted to it in effecting its distillation. 

 Recent experiments with gas-burners have shown that in this 

 direction also there is much room for improvement. 



The amount of light given out by a gas flame depends upon 

 the temperature to which the particles of solid carbon in the 

 flame are raised, and Dr. Tyndall has shown that of the radiant 

 energy set up in such a flame, only the o'-th part is luminous ; the 

 hot products of combustion carry off at lea^t four times as much 

 energy as is radiated, so that not more than one hundredth part 

 of the heat evolved in combustion is converted into light. This 

 proportion could be improved, however, by increasing the tem- 

 perature of combustion, uhiehmay be effected either by intensi- 

 fied air currents or by regenerative action. Supposing that the 

 heat of the products of combustion could be communi- 

 cated to metallic surfaces, and be transferred by conduction 

 or otherwise to the atmospheric air supporting combustion in the 

 flame, we should be able to increase the temperature accumu- 

 latively to any point within the limit of dissociation ; this limit 

 may be fixed at about 2,300° C, and cannot be very much below 

 that of the electric arc. At such a temperature the proportion 

 of luminous rays to the total heat produced in combustion would 

 be more than doubled, and the brilliancy of the li^ht would at 

 the same time be greatly increased. Thus improved, gas-lighting 

 may continue its rivalry with electric lighting both as regards 

 economy and brilliancy, and such rivalry must necessarily result 

 in great public advantage. 



In the domestic grate radiant energy of inferior intensity is 

 required, and I for one do not agree with those who would like 

 to see the open fireplace of this country, superseded by the con- 

 tinental stove. The advantages usually claimed for the open 

 fireplace are, that it is cheerful, " pokable," and conducive to 

 ventilation, but to these may be added another of even greater 

 importance, viz., that the radiant heat which it emits parses 

 through the transparent air without warming it, and imparts heat 

 only to the solid walls, floor, and furniture of the room, which 

 are thus constituted the heating surfaces of the comparatively 

 cool air of the apartments in contact with them. In the case of 

 stoves the heated air of the room causes deposit of moisture upon 

 the walls in heating them, and gives rise to mildew and germs 

 injurious to health. It is, I think, owing to this circumstance 

 that upon entering an apartment one can immediately perceive 

 whether or not it is heated byian open fireplace ; nor is the un- 

 pleasant sensation due to stove-heating completely removed by 

 mechanical ventilation ; there is, moreover, no good reason why 

 an open fireplace should not be made as economical and smoke- 

 less as a stove or hot-water apparatus. 



In the production of mechanical effect from heat, gaseous fuel 

 also presents most striking advantages, as will appear from the 

 following consideration. When we have to deal with the ques- 

 tion of converting mechanical into electrical effect, or vice Zlcrsd, 

 by means of the dynamo-electrical machine, we have only to 

 consider what are the equivalent values of the two forms of 

 energy, and w hat precautions are necessary to avoid losses by 

 the electrical resistance of conductors and by friction. The 

 transformation of mechanical effect into heat involves no losses 

 except those resulting from imperfect installation, and these may 

 be so completely avoided that Dr. Joule was able by this method 

 to determine the equivalent values of the two forms of energy. 

 But in attempting the inverse operation of effecting the conver- 

 sion of heat into mechanical energy, we find ourselves con- 

 fronted by the second law of thermo-dynaniics, which says that 

 whenever a given amount of heat is converted into mechanical 

 eflect, another but variable amount descends from a higher to a 

 lower potential, and is thus rendered unavailable. 



In the condensing steam engine this waste heat comprises that 

 communicated to the condensing water, whilst the useful heat, or 

 that converted into mechanical effect, depends upon the differ- 

 ence of temperature between the boiler and condenser. The 

 b iler pressure is limited, however, by considerations of safety 

 and convenience of construction, and the range of working tem- 

 perature rarely exceeds 120 C. except in the engines constructed 

 by Mr. Perkins, in which a range of 160 C, or an expansive 

 action commencing at 14 atmospheres, has been adopted with 

 considerable promise of snccess, as appears from an able report 

 on this engine by Sir Frederick Bramwell. To obtain more 



advantageous primary conditions we have to turn to the caloric 

 or gas engine, because in them the co-efficient of efficiency 



expressed by — may be greatly increased. This value would 



reach a maximum if the initial absolute temperature T could be 

 raised to that of combustion, and T' reduced to atmospheric 

 temperature, and these maximum limits can be much more 

 nearly approached in the gas engine worked by a combustible 

 mixture of air and hydro-carbons than in the steam engine. 



Assuming, then, in an explosive gas-engine a temperature of 

 1,500° C, at a pressure of 4 atmospheres, we should, in accord- 

 ance with the second law of thermo-dynauiics, find a temperature 

 after expansion to atmospheric pressure of 600° C, and therefore 

 a working range of 1500° - 600 5 = 900°, and a theoretical 



9°° 

 efficiency __-- = about one-half, contrasting very favour- 



ably with that of a good expansive condensing steam-engine, 

 in which the range is 150-30=120° C, and the efficiency 



= ~. A good expansive steam-engine is therefore 



150 + 274 7 



capable of yielding as mechanical workfth part of the heat com- 

 municated to the boiler, which does not include the heat lost by 

 imperfect combustion, and that carried away in the chimney. 

 Adding to these, the losses by friction and radiation in the engine, 

 we find that the best steam-engine yet constructed does not yield 

 in mechanical effect more than 7-th part of the heat energy 

 residing in the fuel consumed. In the gas-engine we have also 

 to make reductions from the theoretical efficiency, on account of 

 the rather serious loss of heat by absorption into the working 

 cylinder, which has to be cooled artificially in order to keep its 

 temperature down to a point at which lubrication is possible; 

 this, together with frictional loss, cannot be taken at less than 

 one-half, and reduces the factor of efficiency of the engine 

 to sjth. 



It follows from these considerations that the gas or caloric 

 engine combines the conditions most favourable to the attainment 

 of maximum results, and it may reasonably be supposed that the 

 difficulties still in the way of their application on a large scale 

 will gradually be removed. Before many years have elapsed we 

 may find in our factories and on board our ships engines with a 

 fuel consumption not exceeding one pound of coal per effective 

 horse power per hour, in which the gas producer takes the place 

 of the somewhat complex and dangerous steam boiler. The 

 advent of such an engine and of the dynamo-machine must mark 

 a new era of material progress at least equal to that produced by 

 the introduction of steam power in the early part of our century. 

 Let us consider what would be the probable effect of such an 

 engine upon that most important interest of this country — the 

 merchant navy. 



According to returns kindly furnished by the Board of Trade 

 and Lloyds' Register of Shipping, the total value of the merchant 

 shipping of the United Kingdom may be estimated at 

 126,000,000/., of which 90,000,000/. represents steamer having 

 a net tonnage of 3,003,988 tons ; and 36,000,000/. sailing 

 vessels, of 3,6SS,ooS tons. The safety of this vast amount of 

 shipping, carrying about five-sevenths of our total imports and 

 exports, or 500,000,000/. of goods in the year, and of the more 

 precious lives c mnected with it, is a question of paramount im- 

 portance. It involves considerations of the ino.st varied kind : 

 comprising the construction of the vessel itself, and the material 

 employed in building it ; its furniture of engines, pumps, sails, 

 tackle, compass, sextant, and sounding apparatus, the prepara- 

 tion of reliable charts for the guidance of the navigator, and the 

 construction of harbours of refuge, lighthouses, beacons, bells, 

 and buoys, for channel navigation. Vet notwithstanding the 

 combined efforts of science, inventive skill, and practical ex- 

 perience — the accumulation of centuries — we are startled with 

 statements to the effect that during last year as many as 1,007 

 British owned ships were lost, of which fully two-thirds were 

 wrecked upon our shores, representing a total value of nearly 

 10,000,000/. Of these ships 870 were sailing vessels and 137 

 steamers, the loss of the latter being in a fourth of the cases at- 

 tributable to collision, The number of sailing vessels included 

 in these returns being 19,325, and of steamers 5,505, it appears 

 that the steamer is the safer vessel, in the proportion of 4^43 to 

 3'46 ; but the steamer makes on an average three voyage for one 

 of the sailing ship taken over the year, which reduces the 

 relative risk of the steamer as compared with the sailing ship per 

 voyage in the proportion of I3'2g to 3'46. Commercially speak- 

 ing, this factor of safety in favour of steam-shipping is to a great 



