December 7, 1888.] 



SCIENCE 



267 



is carefully attended to, and the most recent improvements which 

 experience could suggest, have been adopted to meet the exigen- 

 cies of street-car service. 



The motors have the regular Sprague mounting, as shown in 

 the illustration, being centred on the axles, and flexibly supported 

 to prevent accident from sudden strain. They are very compact 

 and powerful, and combine the requisites of lightest weight possible 

 with highest efficiency. Without this flexible suspension for both 

 directions of running, it is hard to imagine how a motor can be 

 successfully applied to street-cars and fulfil all conditions of actual 

 experience. 



Only one intermediate is used between the armature pinion and 

 the main gear. The gear upon the intermediate, which meshes 

 into the main gear, is built of vulcanized fibre, making it absolutely 

 noiseless, is so constructed that it is very durable, and it will out- 

 wear the steel teeth which mesh into it. 



All the bearings are self-oiling and completely dust-proof, and 

 should run at least a month with little or no attention, and each is 

 so constructed that it can be removed without dismounting the 

 machine. The brushes are on an entirely new principle and 

 design, and are remarkable for ease of adjustment, and work with 

 ■equal facility in running either forward or backward. By their 

 means a perfect electrical contact is secured, without excessive 

 pressure on the commutator, and all wear is reduced to a mini- 

 mum. 



The whole motor and the gearing, and all parts, are so placed 

 that they can be perfectly shielded and shut in by a tight-fitting 

 cover, so that by this means it is rendered impossible for moisture 

 ■or dust to get into any of the working parts. 



The design and construction of every part of this truck are not 

 only to have each detail as strong as possible, but also to have it 

 readily accessible, and to reduce all care and attendance to a mini- 

 mum. 



We congratulate the Sprague Company upon the success which 

 has attended their installations in the past, and we anticipate for 

 them an increased success in the future. 



SCIENTIFIC NEWS IN WASHINGTON. 



Kow Birds soar: the Conditions of the Atmosphere, and not a Peculiar 

 Structure of the Bird, the Essential Factor. — How Men of All 

 Ages and in Every Country have made of Themselves Beasts of 

 Burden. — How the Navajo gamble. 



The Soaring of Birds. 

 Whoever has watched an eagle or other soaring bird as he 

 ■circles through the air has marvelled that he is able to sustain him- 

 self without the flapping of his wings. Not only does he do this, 

 but he rises higher and higher from the earth, enlarging his circles 

 and seemingly increasing his speed, until he attains so great a 

 height as to be almost invisible. This apparent defiance of the 

 laws of gravitation has long been recognized as a problem to be 

 solved, and many explanations have been offered. The latest con- 

 tribution to the subject is by Mr. G. K. Gilbert, who read a paper 

 to the Philosophical Society of Washington at its last meeting. 

 He concluded by saying that when he proposed to the society's 

 committee to place the paper on its programme, he supposed his 

 theory of soaring to be novel, but that he had since found himself 

 anticipated by Lord Rayleigh, who communicated the same ex- 

 planation to A'ati/re in 1S83 (vol. xxvii. p. 534), and by Mr. Hubert 

 Airy, who independently reached the same result at about the same 

 time {IVit/ure, vol. xxvii. p. 590). It appeared, however, from the 

 informal discussion which followed the reading of the paper, that 

 the earlier presentation of the theory had escaped the attention of 

 many ornithologists and physicists present, and it may therefore 

 not be amiss to restate it in the pages of Scu'/tCt.: The following 

 paragraphs are extracted from Mr. Gilbert's paper. 



" The soaring bird, with wings expanded, is formed so as to move 

 forward with little friction, and downward with great friction. We 

 may conceive him as having two coincident motions, — a forward 

 motion, initiated by muscular action ; and a downward motion, slow 

 but continuous, under the pull of gravity. By variations of the at- 

 titude of his wings and tail, he can and does control the direction 

 of his forward motion. 



" If the forward component of motion is horizontal, the resultant 

 of the two motions is obliquely downward. In order that the re- 

 sultant may be horizontal, it is necessary (i) that the forward 

 component be directed obliquely upward, and (2) that it exceed a 

 certain minimum amount. 



" However small may be the friction created by the forward 

 motion, it is not m'/. It constantly tends to check the motion ; 

 and, unless the energy it consumes is in some way replaced, the 

 forward motion is eventually so reduced that horizontal motion 

 cannot be maintained. 



" It is proposed to show that the needed compensatory energy 

 may be derived from the differential motions of the air. 



" I shall not dwell on the utilization of upward currents of air. 

 It is evident without explanation that when a bird sails through 

 air that is rising, whether vertically or obliquely, he is carried up- 

 ward with the air, and, if the upward motion of the air equals or 

 exceeds the downward motion of the bird under gravity, he does 

 not need to flap his wings in order to sustain himself. But such 

 opportunities are of exceptional occurrence; and, while it is highly 

 probable that they are not neglected, recourse to soaring is too 

 frequent, and with certain species too generally successful, to per- 

 mit us to believe that an upward current is its necessary condition. 

 I shall confine my attention to the less obvious resource of hori- 

 zontal currents. 



" It is frequently observed that the velocity of the wind increases 

 from the ground upward. Let us assume, for simplicity's sake, 

 that the air-currents above and below a certain horizontal plane 

 have the same direction but different velocities, the upper moving 

 the faster by a certain amount, /'. A soaring bird is moving through 

 the lower air in the opposite direction, and the bird's velocity with 

 reference to the air is V. 



" It should be borne in mind that velocity is merely rate of rela- 

 tive motion. Fully to define the velocity of a body, it is neces- 

 sary to state to what other body its motion is referred. In this 

 case the velocity of the upper current with reference to the lower 

 current is i ; the velocity of the bird with reference to the lower 

 current is V,- and, since the bird and the upper current pass the 

 lower current in opposite directions, the velocity of the bird with 

 reference to the upper current is K-f-z'. 



" Now let the bird change his course, turning obliquely upward 

 and passing into the upper current. His velocity with reference to 

 the air in which he is immersed is at once increased from K to 

 V+2. Next let the bird wheel, to the right or to the left, until the 

 direction of his motion is coincident with that of the wind. His 

 velocity with reference to the upper current is still V+i, but the 

 reversal of his direction has changed his relation to the currents. 

 He is passing the lower and slower current more rapidly than he 

 passes the upper, and his velocity with reference to the lower cur- 

 rent is greater by their difference : it is V+2 i. Now let him de- 

 scend obliquely, and enter the lower current. His velocity is not 

 affected by the transfer. It is still K-I-2/, referred to the lower 

 current. Finally let him wheel in the lower current until his direc- 

 tion is once more directly opposed to that of the wind. The cycle 

 of evolutions leaves him with the velocity V+2i, referred to the 

 lower current, in place of his initial velocity V, referred to the same 

 datum. He has gained a velocity 2 z, or double the velocity of one 

 air-current referred to the other, and he has resumed his original 

 relation to the currents. Manifestly he can repeat the process in- 

 definitely. 



" Add now that the velocity thus gained is the required compen- 

 sation for the velocity lost by friction, and the essence of the theory 

 is stated." 



Mr. Gilbert then proceeded to pass from the special case as- 

 sumed for the sake of simplicity to the more general case, pointing 

 out that certain assumptions which facilitated the statement of the 

 theory were not essential to the analysis. Provided the air in the 

 region traversed by the bird has some differential motion in a hor- 

 izontal sense, and provided the bird regulates his circling course 

 so as to ascend when his direction of flight is opposed to the direc- 

 tion of the differential motion of the air into which he rises, and so 

 as to descend when the relations are reversed, he will acquire from 

 the differential motion of the air an acceleration of his own velo- 

 city. If this acceleration is less than the concurrent loss by fric- 



