io8 



NATURE 



[December 3, 1891 



The exciting current is led into the field magnet by the 

 novel employment of the two endless metallic cords seen 

 to the left of Fig. 33, which saves the necessity of using 

 a standard to carry contact brushes, and the smallness 

 of the power spent in exciting the field magnet, com- 

 pared with the power developed by the machine, is seen 

 from the dwarf-like character of the direct current exciting 

 dynamo in Figs. 33 and 34.^ 



This three-phase alternate current .dynamo of Mr. 

 Brown's, on account of the simphcity and solidity of its 

 design, the slow speed of its rotation, and the entire 

 absence of the experimental makeshifts which are sup- 

 posed to be characteristic of an electrician, but which are 

 in reality evidences of the rapid development of his 

 tools, appeals especially to the mechanical engineer. 

 It is therefore probable that the employment of so 

 well constructed a dynamo at Lauften, and so smoothly 

 running a motor at Frankfort, will bring home to the 

 mechanical engineer that he can now avail himself, for 

 the practical transmission of power, of that silent carrier 

 electricity — a carrier which, while it can communicate 

 a great force almost instantaneously to a vast distance 

 through a thin wire, travels itself so leisurely that, in its 

 steady flow, it experiences no extra difficulty whether it 

 goes up hill or down dale, overhead or underground, in 

 a straight line or round a succession of sharp corners. 



EXPERIMENTS IN AERODYNAMICS?' 



'X'HE subject of this memoir is of especial interest at 

 -*■ the present time, when the skill of a distinguished 

 inventor is understood to be engaged in attacking the 

 many practical difficulties which lie in the way of arti- 

 ficial flight upon a large scale. For a long time the re- 

 sistance of fluids formed an unsatisfactory chapter in our 

 treatises on hydrodynamics. According to the early 

 suggestions of Newton, the resistances are(i) proportional 

 to the surfaces of the solid bodies acted upon, to the 

 densities of the fluids, and to the squaresof the velocities ; 

 while (2) " the direct impulse of a fluid on a plane surface 

 is to its absolute oblique impulse on the same surface as 

 the square of the radius to the square of the sine of the 

 angle of incidence." The author of the work^ from 

 which these words are quoted, in comparing the above 

 statements with the experimental results available in his 

 time (1822), remarks : — "(i) It is very consonant to ex- 

 periment that the resistances are proportional to the 

 squares of the velocities. ... (2) It appears from a com- 

 parison of all the experiments, that the impulses and 

 resistances are very nearly in the proportion of the 

 surfaces. ... (3) The resistances do by no means vary 

 in the duplicate ratio of the sines of the angle of in- 

 cidence." And he subsequently states that for small 

 angles the resistances are more nearly proportional to 

 the sines of incidence than to their squares. 



It is probable that the law of velocity tended to support 

 in men's minds the law of the square of the sine. For, if 

 both be admitted, it follows that the resistance, normal to 

 the surface, experienced by a plane when immersed in a 

 stream of fluid, depends only upon the component of the 

 velocity perpendicular to the surface. That the effect 

 should be independent of the component parallel to the 

 plane seems plausible, inasmuch as this component, if it 

 existed alone, would exercise no pressure ; but that such 

 a view is entirely erroneous has been long recognized by 

 practical men, especially by those concerned in naviga- 

 tion. 



From the law of the simple sine, enunciated by Robi- 

 son, it follows at once that the pressure upon a lamina 



1 We are indebted to Industries and the Electrician for some of the 

 illustrations used in this article. 



* " Experiments in Aerodynamics." By S. P. Langley. "Smithsonian 

 Contributions to Knowledge." (Washington, 1891.) 



3 " System of Mechanical Philosophy," by John Robison, vol. ii., 1822. 



exposed perpendicularly to a stream may be increased to 

 any extent by imparting to the lamina a sufficiently high 

 velocity in its own plane. The immense importance of 

 this principle was clearly recognized by Mr. Wenham in 

 his valuable paper upon flight ; ^ and a few years later the 

 whole subject was discussed by the greatest authority 

 upon such matters, the late Mr. W. Froude, with 

 characteristic insight and lucidity. ^ 



The theoretical problem of determining the resistance 

 from the first principles of hydrodynamics is not free from 

 difficulty, even in the case of two dimensions, where a 

 long rectangular lamina is exposed obliquely to a stream 

 whose direction is perpendicular to the longer sides. The 

 formula ^ resulting from the theory of Kirchhoff, viz. 



pvs 



(I) 



■ir sm a 

 4-}- IT sin a 



where p is the density of the fluid, and V is the total 

 velocity of the stream flowing at the angle a with the 

 plane of the lamina, shows that when a is small the re- 

 sistance is nearly proportional to sin a. Moreover, (i) 

 agrees with the experiments of Vince.'* 



It will be seen that the laws of resistance were fairly 

 well established many years ago, at least in their main 

 outlines. Nevertheless, there was ample room for the 

 systematic and highly elaborate experiments recorded in 

 the memoir whose title stands at the head of this article. 



The work appears to have been executed with the skill 

 and thoroughness which would naturally be expected of 

 the author, and will doubtless prove of great service to 

 those engaged upon these matters. The scanty reference 

 to previous knowledge, which Prof. Langley holds out 

 some promise of extending in subsequent publications, 

 makes it rather difficult to pick out the points of greatest 

 novelty. The main problem is, of course, the law of 

 obliquity, and this is attacked with two distinct forms of 

 apparatus. The general character of the results, exhibited 

 graphically on p. 62, will be made apparent from the ac- 

 companying reproduction, in which are added a curve D, 



' Report of Aeronautical Society for 1866. 



^ Proc. Inst. Civ. Eng., 1871 (discussion upon a paper by Sir F. Knowles\ 



3 See Phil. Mag., December 1876. Also Basset's "Hydrodynamic<," 

 vol. i. p 131. 



4 Phil. Irans., itqS. 



NO. 1 153, VOL. 45] 



