December 3, 1891] 



NATURE 



109 



corresponding to (i), and E, representing the law of sin^a. 

 In each case the abscissa is the angle a, and the ordinate 

 is the normal pressure, expressed as a percentage of that ex- 

 perienced when a = 90^ Of Prof. Langley's curves, A re- 

 lates to a square plane 12 inches x 12 inches, B to a rect- 

 angle 6 X 24 inches, and C to a rectangle 30 x 4*8 inches, 

 the leading edge (perpendicular to the stream) being in each 

 case specified first, so that the theoretical curve D cor- 

 responds most nearly to C. It will be seen at a glance 

 that at small angles the pressure is enormously greater 

 than according to the law of sin- a. The differences 

 between A, B, C, anticipated in a general manner by 

 Wenham and Froude, are of great interest. They 

 demonstrate that in proportion to area a long narrow 

 wing is more efficient as a support than a short wide one, 

 and that in a very marked degree. 



Up to a certain point there is no difficulty in giving a 

 theoretical account of these features. When a rectangular 

 lamina is exposed perpendicularly, there is one point, i.e. 

 the centre, at which the velocity of the stream is annulled. 

 At this point the pressure attains the full amount, ipV^, 

 due to the velocity of the stream, while at every other 

 point the pressure is less, and falls to zero at the 

 boundary. If the lamina is sloped to the stream, as in 

 B and C, there is still a median plane of symmetry ; and 

 at one point in this plane, but now in advance of the 

 centre, the full pressure is experienced. In strictness, 

 there is only one point of maximum pressure, whatever 

 may be the proportions of the lamina. But if the rectangle 

 be very elongated, there is practically a great difference 

 in this respect according to the manner of presentation, 

 although the small angle o be preserved unchanged. For 

 when the long edges are perpendiculTJr to the stream (C), 

 the motion is nearly in two dimensions, and the region 

 of nearly maximum pressure extends over most of the 

 length. But the case is obviously quite different when 

 it is the short dimension that is perpendicular to the 

 stream, for then along the greater part of the length there 

 is rapid flow, and consequently small pressure. 



It will naturally be asked whether any explanation can 

 be offered of the divergence of C from the theoretical 

 curve D. This is a point well worthy of further experi- 

 ment. It seems probable that the cause lies in the 

 suction operative, as the result of friction, at the back of 

 the lamina. That the suction is a reality may be proved 

 without much difficulty by using a hollow lamina, AB 

 (Fig. 2), whose interior is connected with a manometer. 



If there be a small perforation at any point C, the mano- 

 rneter indicates the pressure, positive or negative, exer- 

 cised at this point, when the apparatus is exposed to a 

 blast of air. 



When once the law of obliquities is known, the problem 

 of aerial maintenance presents no further theoretical 

 difficulty. It was successfully treated many years ago by 

 Penaud,' and somewhat later by Froude, whose interest- 

 ing letters, written shortly before his death, have recently 

 been published.^ In perhaps the simplest form of the 



' See Report of Aeronautical Society for 1876. 

 ' Edinburgh Proceedings, R. E. Froude, 1891. 



NO. 1 153, VOL. 45] 



question the level is supposed to be maintained with the 

 aid, e.g., of screw propulsion, the necessary maintenance 

 being secured by an aeroplane slightly tilted (a) upwards 

 in front. The work required to be expended in order to 

 maintain a given weight depends upon the area of plane, 

 the inclination, and the speed. Penaud's results show 

 that, if skin friction could be neglected, the necessary work 

 might be diminished indefinitely, even with a given area 

 of wing. For this purpose, it would only be necessary to 

 increase the speed and correspondingly to diminish a. 

 But when skin friction is taken into account, the work 

 can only be reduced to a minimum, and to do this with 

 with a given area of wing requires a definite (large) 

 velocity, and a definite (small) inclination. The accurate 

 determination of the tangential, as well as of the normal, 

 force experienced by an inclined plane is thus of essential 

 importance in the question of flight. 



The work of Penaud seems to be so little known that 

 it has been thought desirable to recapitulate some of his 

 theoretical conclusions. But we owe to Penaud not 

 merely sound theory, but the actual construction of a 

 successful flying machine, in which horizontal flight is 

 maintained by a screw propeller. In these models the 

 energy is stored by means of stretched india-rubber, a 

 method available only upon a small scale. It is probable 

 that the principle of the rocket might be employed with 

 advantage ; and even upon a large scale the abolition of 

 all machinery would allow of considerable extravagance 

 in the use of explosive material. This method is espe- 

 cially adapted to the very high speeds which on other 

 grounds are most suitable. 



In the chapter on " The Plane Dropper," some striking 

 experiments are described, illustrating the effect of a for- 

 ward movement in retarding the fall of a horizontal plane. 

 Prof. Langley seems hardly to recognize that there is 

 nothing really distinctive in this arrangement when he 

 says : — 



"It is, of course, an entirely familiar observation that 

 we can support an inclined plane by moving it laterally, 

 deriving our support in this case from the upward com- 

 ponent of pressure derived from the wind of advance ; 

 but, so far as I am aware, this problem of the velocity of 

 fall of a horizontal plane moving horizontally in the air 

 has never been worked out theoretically or determined 

 experimentally, and I believe that the experimental in- 

 vestigation whose results I am now to present is new." 



But, apart from the complications which attend the 

 estabhshment of a uniform regime, there is no essential 

 difference between the two cases. The hydrodynamical 

 forces depend only upon the magnitude of the relative 

 velocity and upon the inclination of this relative velocity 

 to the plane. All else is a question merely of ordinary 

 elementary mechanics. 



It is interesting to note that Prof. Langley's ex- 

 perience has led him to take a favourable view of the 

 practicability of flight upon a large scale Such was also 

 the opinion of Penaud, who (in 1876) expresses his 

 conviction "that, in the future more or less distant, 

 science will construct a light motor that will enable us to 

 solve the problem of aviation." But sufficient maintain- 

 ing power is not the only requisite ; and it is probable 

 that difficulties connected with stability, and with safe 

 alighting at the termination of the adventure, will exercise 

 to the utmost the skill of our inventors. 



Rayleigh. 



PRELIMINARY NOTICE OF A NEW 

 BRANCHIATE OLIGOCH^TE. 

 'T'HE term "Ann^lides abranches sdtig^res," applied 

 ^ by Cuvier to the group which included the ter- 

 restrial and fresh-water Annelids, now known as the 

 Oligochaeta, is no longer applicable to that group. 

 Several Oligochaeta have been described as possessing 



