122 



NATURE 



\yune 9, 1881 



it would be impossible to make a machine that would 

 travel on common roads and even mount hills, but " even 

 suppose that such a machine could be constructed to 

 carry twenty or thirty passengers at ten miles an hour, 

 put it on a level railway and it would carry 200 or 300 

 passengers at thirty or forty miles an hour." 



In his first colliery railways at Killingworth and Hetton 

 he laid the lines in a series of flat reaches separated by 

 inclines, and working the inclines by fixed engines, con- 

 fined the functions of the locomotive to drawing the 

 waggons along the flat reaches. It was this insistence 

 on the level road that enabled him to use smooth wheels, 

 and not that he had discovered any adhesion previously 

 unknown or that others had overlooked. 



Stephenson's position was a nearly level line at any 

 price on wiich the adhesion of the wheels is sufficient as 

 against a road following the slope of the country, for 

 which, according to his view, by whatever means the 

 adhesion might be increased, the iron horse was ill adapted. 

 In the clear conception of the importance of this level 

 road, coupled with his determinate insistence in carrying 

 out his view, no matter what the difficulty — the veritable 

 removal of mountains — have we not the best of all proof 

 that, however unconsciously, he was guided by a percep- 

 tion of that law which connects the limits in size and 

 activity of structures, with the strength of the material 

 of which they are composed. And by which law we 

 may now perceive that it is only by smoothing the road 

 and so reducing the call for strength and power that 

 we have made our machines to exceed in size and speed 

 the limits which Nature had reached in her animals. 



Into another law, called the Conservation of Energy, 

 there can be no doubt that Stephenson had an insight 

 far beyond his time. He saw that the conveyance of 

 a load was not a question of force, but of the product 

 of force into the distance traversed, and that however 

 great might be the tractive power of his engine, its 

 speed must depend on the ratio of the rate at which 

 steam could be generated to the load. So long there- 

 fore as the tractive power was so large as — compared with 

 the steam-generating power of his boiler — to prevent his 

 engine, when fully loaded, travelling at more than ten 

 miles an hour, he could gain nothing by increased ad- 

 hesion. But, on the other hand, in his first engine the 

 desideratum was increased steam-generating power for 

 the same weight of boiler. 



With, as Robert Stephenson has told us, the direct 

 object of accomplishing this, George Stephenson turned 

 the exhaust steam in the form of a jet or blast up the 

 chimney of his second locomotive, " Puffing Billy." If 

 this is so, and there appears no evidence to the contrary 

 it was a prediction with regard to the motion of fluids, 

 for the making of which there is as yet no established law 

 in the theory of hydrodynamics. That the result is such 

 as was here predicted, or that a jet of steam or of air 

 playing at high velocity along the interior of an open- 

 ended tube does impart motion to the air within the tube 

 and causes a current, is of course now well known, but 

 our present knowledge is derived from the experience of 

 the locomotive chimney. There is no evidence that it 

 was known to any one before 1S15, nor indeed has there 

 been found any other mechanical purpose of general 

 importance in which the same action could be usefully 



employed. Neither in the stationary engine nor yet in 

 the marine engine has it proved economical. Thus the 

 locomotive and its offspring, the portable engine, were 

 the only machines possessing this organ. 



Although it has been the custom for writers on the 

 steam-engine to speak as though the manner of action of 

 the blast were self-evident, this only shows that these 

 authors have not understood it — indeed how should they ? 

 The general law on which the action of the blast depends 

 is that a jet of fluid issuing into surrounding fluid at rest 

 will not, when it has more than a certain velocity, proceed 

 in a straight vein or column, but begins at once to wriggle, 

 and as it advances involves itself in an extremely complex 

 manner with the surrounding fluid, with which it shares 

 its forsvard momentum. It is only during the last few 

 years that the generality of this action and the circum- 

 stances on which it depends have attracted attention, and 

 the completeness with which the action has been over- 

 looked is shown by the numerous attempts that have been 

 made to invent fanciful explanations of the following 

 phenomenon. When a jet of steam, say half an inch in 

 diameter, issues from a high-pressure boiler, as from a 

 gauge cock, although the steam itself must have the 

 temperature of boiling water, still the hand may be held 

 in the jet at a distance of two or three inches from the 

 cock without any inconvenience. How has the tempera- 

 ture of the steam become lowered ? is the question for the 

 answering of which numerous hypotheses have up to quite 

 recently been invented. The answer is that the tempera- 

 ture of the steam does not become lowered, any more 

 than the strength of the mustard in a sandwich, but that 

 the steam has involved within its column layers of cool 

 air, sandwich fashion, and as the combination rapidly 

 passes the same point of the skin, the sensation produced 

 is that of the mean temperature of the air and steam. 



It is on this action of a jet to mix itself up with the 

 surrounding medium that the draught produced by the 

 blast up the chimney depends, and Stephenson's con- 

 fident prediction of this draught is the best evidence 

 that observation had led him to a perception of the more 

 general action. 



Considering the capacity of the man as shown by his 

 other work, it would have been a matter for surprise had 

 not Stephenson acquired a unique knowledge of the phe- 

 nomena of fluid motion. He had the best opportunity for 

 observation — his whole time had been spent in the care 

 of pumps, pumping-engines, and the arrangements for 

 ventilating and draining mines. His habit was to bring 

 all his ideas at once to the test of experiment ; and in 

 devising his safety-lamp he had carried out a very careful 

 series of experiments on the behaviour of jets and the 

 rate of their admi.xture with the surrounding air. 



Although, as shown by the employment of the mvflti- 

 tubular boiler, Stephenson's mechanical insight does not 

 perhaps stand out in so very clear a light, inasmuch as he 

 made this step at the suggestion of Mr. Booth, still it 

 cannot have been fortuitous that in adopting these small 

 tubes he should have at once introduced all those condi- 

 tions on which their employment is alone successful. 



That small tubes of the same aggregate capacity as a 

 single flue afford greater cooling surface for the hot gases 

 is indeed obvious, but it was Stephenson's own observa- 

 tion that taught him that such increase was desirable, 



