28 



NA TURE 



[May 10, 1 88 •■ 



The adhesion between metallic contacts consequent upon the 

 passage of a current has been carefully investigated by Mr. 

 Stroh, who observed it in the case of all of a great number of 

 metals with which he experimented. My first observations on 

 the subject (one of which is mentioned in the paper) were made 

 with the refractory metal platinum, and not with bismuth, as the 

 writer of the note seems to infer; and though Mr. Stroll's ex- 

 planation — that the adhesion is due to fusion — is quoted, I 

 express no opinion of my own on the matter. Whatever may 

 be the cause, it seems evident enough that such adhesion must 

 necessarily be detrimental to the perfect action of a microphone, 

 1 1 I am not aware that attention has been previously 

 directed to this point. 



It is not correct to attribute to me the opinion, as stated in 

 the note, "that the heat generated by the current plays an 

 important part, for in carbon this reduces the resistance, whilst 

 in metals it increases it." On the contrary I give reasons for 

 believing that at least a moderate degree of heat increases the 

 resistance of loose carbon contacts. Increased current, how- 

 ever, is accompanied by diminished resistance, and while I 

 am not prepared to say that heat plays no part whatever in the 

 matter, it appears to me probable that the effect is mainly owing 

 to some other incident of the stronger current, e.g. greater 

 difference of potential. 



My experiments on metals were not, as the writer supposes, 

 entirely confined to bismuth. More than a hundred observations 

 were recorded of the resistance of platinum and copper contacts 

 under different conditions, and some of these are referred to in 

 the paper. Owing, however, to the low specific resistance of 

 these metals, the methods which I had applied with success in 

 the case of carbon were found to be unsuitable, and the results 

 obtained, though not on the whole inconsistent with those 

 yielded by bismuth, were unsatisfactory and inconclusive. Bis- 

 muth was chosen for the bulk of the experiments, principally on 

 account of its bad conductivity, which renders changes in the 

 resistance of the contact easier of observation ; but since it was 

 my object to contrast the behaviour of metals with that of carbon 

 (which is infusible), its ready fusibility is another advantage. If 

 I had desired to make a good metallic microphone, I should 

 probably have thought with the writer of the note that bismuth 

 was "the very metal which ought to have been avoided." But 

 for experiments conducted with the object of ascertaining the 

 causes of the generally recognised fact that metals, as a class, 

 are inferior in microphonic efficiency to carbon, it is evident 

 that the metal which gives the poorest microphonic effects is the 

 very one which ought to be selected, on account of the probability 

 that with such a metal these causes would be most conspicuous. 



As a matter of strict scientific exactness I agree with the writer 

 that "no conclusion of any value as to metals in general can be 

 drawn from experiments on bismuth alone." But since the 

 physical properties with respect to which bismuth differs from 

 carbon, and which have any probable connection with micro- 

 phonic action, seem to be common in various degrees to all 

 metallic bodies, I venture to predict with tolerable confidence, 

 that if the experiments described in the paper are repeated with 

 suitable apparatus, it will be found that all the conclusions 

 arrived at with regard to bismuth (as summarised in the abstract 

 before referred to) are also true to a greater or less extent for 

 any other ordinary metal. SHELFORD Bidwell 



Wandsworth, April 22 



[The necessary brevity of the note to which Mr. Bidwell refers 

 precluded lengthy quotations. At the same time it was only 

 natural to draw attention to the weak point in Mr. Bidwell's 

 argument, namely, that the behaviour of the metals generally could 

 not with any certainty be argued from observations made, as Mr. 

 Bidwell aduiils, on the very metal which for practical ends 

 t nght tube avoided, It is greatly to be wished that Mr. Bidwell 

 will so far further improve the capabilities of his apparatus as 

 not only to be able to get conclusive results with other metals, 

 but also so as to enable him to say why his apparatus gave results 

 that were unsatisfactory and inconclusive with good conducting 

 metals such as platinum and copper.] 



The Soaring of Birds 



For mr re than twenty years I have watched with admiration 

 the ; oaring of the black vulture of Jamaica (I'ultur aura). 

 When once well up in the air it rarely moves its wings, except 

 to change the direction of its flight. It can soar whenever there 

 is even a light wind. 



I entirely concur with Mr. Hubert Airy in the main point of 

 his general conclusion, as given in vol. xxvii. p. 592. "Varia- 

 tions in the strength and direction of current " can, as he says, 

 be so "utilised" by birds as to enable them to soar. But a 

 high wind is not necessary ; and a downward current, even when 

 approaching the perpendicular, may, if of sufficient velocity, be 

 utilised. 



Whenever there is a wind there will be ascending and descend- 

 ing currents in some places. This will be so even in a level 

 plain which presents no considerable obstacles, such as trees or 

 buildings, to the stream of air. The plain will be bounded by 

 hills of varying height, and it will vary in breadth. A stream 

 of water would merely flow more rapidly through the narrower 

 channels ; but a stream of air, being highly elastic, will also rise 

 and fall, and it will have its eddies in planes more or less 

 inclined to the horizon, and will often acquire a rolling motion. 

 Assuming the existence of ascending and descending currents, 

 the soaring is a very simple matter. The bird contrives to 

 remain much longer in the upward currents than in the down- 

 ward. It will glide along the ascending side of a wave of air 

 and cut across the descending side. It will make many spiral 

 turns in an ascending current of sufficient amplitude. I have 

 often seen the vulture ascend thus for more than 2000 feet, 

 keeping near a steep mountain side. If the bird encounters a 

 descending current, of which it is instantly aware through the 

 diminished pressure on its wings, it will either wheel to the right 

 or left to get out of it, or, altering the pitch of its wings, will 

 descend swiftly so as to acquire the necessary impetus for a rapid 

 escape, or will do both. 



It can also avail itself of inequalities in the velocity of hori- 

 zontal currents flowing parallel to one another at the same eleva- 

 tion. The bird, let us suppose, encounters a strong horizontal 

 current, as warm as it is rapid, issuing from a mountain valley 

 or a cutting through a forest. Instantly throwing its wings inti 

 a plane nearly vertical, it receives on them the force of the 

 current, and in a few seconds acquires its velocity. Pitching its 

 wings also for a downward flight it shoots quickly through th< 

 current, having acquired a speed more than sufficient for th< 

 recovery of its original elevation. If the current be very strong 

 and very narrow, it need not be horizontal, but may approach 

 the perpendicular. The bird will not remain in it long enougl 

 to be carried far down, while it acquires an impetus much more 

 than compensating for the slight loss of elevation. It must be 

 remembered that when the bird is gliding at a high rate o! 

 speed, the resistance of the air, through its inertia, to any move 

 ment except in the plane of the wings, almost equals that of I 

 solid body, and a change of direction causes a very slight loss o 

 momentum. 



What rapidity of currents is necessary for soaring must depenc 

 in great measure on the structure of the bird. The vulture is, 1 

 believe, comparatively heavy, but I think that, having onc< 

 acquired speed by a short and steep descent, it can glidi 

 through still air (or at right angles through air having a unifor; 

 horizontal notion) at the rate of twenty miles an hour, descending 

 not more than one in twenty. If, therefore, the bird could b 

 always in an upward current of only one mile an hour, it could 

 maintain itself in the air. A gentle breeze of ten miles an hour, 

 with one mile an hour of ascent — and a much steeper ascent thai 

 this must be frequent enough where there are hills — would suffii 

 to sustain the bird ; and as an average of ten miles an hour im 

 plies local or occasional gusts of greater velocity, of which the 

 bird knows how to avail itself, it could ascend in such a current, 

 and so be able to work to windward. If besides hills of 

 moderate inclination, there are also trees, walls, houses, the air 

 will often be driven upwards, vertically or nearly so, with as 

 great or even greater speed than that of its average horizontal 

 movement ; and of this upward movement the birds avail them- 

 selves most skilfully. I have frequently seen the vultures 

 working their way thus against a high wind. Their movements 

 are very irregular. Sometimes, to avoid a violent gust, they will 

 drop almost perpendicularly to within a yard or two of the 

 ground, and shooting abruptly sideways with the high velocity 

 gained by the drop, will get into an upward current in which, if 

 ample enough, they will wheel, or else will cross and recross it, 

 till they have gained a sufficient elevation, and then, taking 

 advantage of a lull, will glide to windward. 



With a breeze of only five miles an hour, there will be in 

 many places upward currents of high inclination caused by the I 

 usual irregularities of surface. Keeping sometimes in these and 

 sometimes in currents more slightly ascending, for, say, two- 



