364 



NATURE 



[March II, 1875 



existence of the premisses do not practically obtain. I may 

 remark, however, that Mr. Prideaux does not show how or in 

 what manner my arguments are inapplicable, but contents him- 

 self with pointing out what he imagines to be an error in my 

 conception of the mechanism of the part in question. Now, I 

 candidly confess that my knowledge of the state of things at the 

 base of the aorta was not based upon practical observation, but 

 at the same time I must, in justice to myself, say that in the 

 mental review which I took of the possibilities of construction of 

 the valves, I recognised the probable existence of the case which 

 forms the subject of Mr. Prideaux's demonstration. But as he 

 seems to think that if this error be granted the whole reasoning 

 which follows is consequently invalid, I assert that it is by no 

 means obviously certain, a priori, that an alteration in the condi- 

 tions of its application must necessarily modify the conclusion. 

 On the contrary, this very point which he deems it needless to 

 prove because he has no doubt that it will be allowed, is the 

 very point on which the whole question turns. I think also that 

 in the further illustration of this I shall be able to show that Mr. 

 Piideaux has missed the sole idea for which I was anxious to 

 contend, viz., "that no mechanical advantage is gained by the 

 expansion of the aorta towards its termination." Moreover, if I 

 can point out the occasion of his difference from myself, I shall 

 at the same time be rendering my own assurance the more com- 

 plete. 



In the first place, then, I think the difference is more verbal 

 than real, and depends upon a certain ambiguity in the term 

 "force of reflux." This I have interpreted to mean the pressure 

 which would be represented by an area equal to the normal 

 calibre of the vessel, being of opinion that it cannot naturally be 

 applied to the multiplied pressure which would be given by 

 taking the total area of expansion as its equivalent. The former 

 pressure is transmitted without diminution to the unsupported 

 area of the valves. 



Again, the statement that "owing to the expansion of the 

 aorta towards its termination, the force of reflux is most efficiently 

 sustained by the muscular substance of the ventricle," is undoubt- 

 edly true in one sense ; but in this case it is reduced to a mere 

 truism, and amounts simply to this, that "the muscidar suhs'ance 

 of the ventricle being partially exposed to the contact of the 

 column of blood, the latter rests upon it," and this, indeed, 

 holds good whether the valves be mediate between the blood 

 and the structure of the ventricle or not However, I cannot 

 help crediting the enunciation of Mr. Savory's theory with more 

 than this, and maintain that it naturally induces the idea that 

 the arrangement is in some way advantageous to the valves, 

 i.e., that the pressure is lessened on the unsupported portion. 



That this conclusion was contrary to mechanical laws was what 

 I endeavoured to show in my first letter, and that my arguments 

 are equally applicable in the present instance is evident from the 

 fact that the existence of that portion of the valves which rests 

 upon the ventricle is mechanically unimportant and need not be 

 considered, since tlie remainder of their surface bears just the 

 same pressure as if they were attached directly to the margin of 

 the ventricular ring. 



It is possible, however, to make one other supposition on 

 behalf of Mr. Savory's theory, that the error lies in its statement, 

 and not in the theory itself. If this be the case it would at any 

 rate be much better expressed thus : " That though the aorta ex- 

 pands towards its termination, the increase of pressure which the 

 valves would thus have to bear is compensated by the support 

 which they receive from the muscular substance of the ven- 

 tricle." 



With regard to the last paragraph of your correspondent's 

 letter, in which he denies the possibility of contraction of the 

 aortic orifice during the diastole, I can only say that instead of 

 imagining this to be the case, I expressed a strong doubt as to 

 its occurrence. For the original statement the text-book and not 

 myself is responsible, as may be seen from the following quota- 

 tion : " The reflux of blood is most efficiently sustained by the 

 ventricular wall, which at the moment of its occurrence is pro- 

 bably in a state of contraction." That this, however, should take 

 place is, as Mr. Prideaux justly observes, an impossibility, and 

 only proves the existence of another error either of theory or 

 enunciation. W. Percy Ashe 



51, Palace Gardens Terrace 



Flight of Birds 

 The Duke of Argyll appears to maintain that a bird can 

 remain at rest in a uniform horizontal current by simply 



placing and maintaining itself in a certain fixed attitude. * He 

 seems likewise to think that the muscular effort required to 

 maintain this attitude is somehow an explanation of the pheno- 

 menon. 



But would a dead bird, of precisely the same weight, size, shape, 

 &c., rigidly fixed in the same attitude, also remain poised under 

 like conditions ? Of course I do not refer to the practical diffi- 

 culty of maintaining an exact balance in the case of a dead bird, 

 but in order to test the theory I suppose a mathematically uni- 

 form current and a mathematically perfect poise. 



The live bird being perfectly motionless, the two would be 

 precisely in the same mechanical condition, although the rigid 

 attitude of the live bird would be maintained by dint of muscular 

 exertion, and that of the dead bird by rigor mortis. Under 

 these circumstances, would the dead bird fall to the ground or 

 remain poised ? If the former, what mechanical forces would 

 apply to it which do not apply to the live bird ? If the latter, 

 then it would clearly follow that both birds could without change 

 of attitude move with a uniform velocity, in a horizontal line, 

 through still air ; for it is clear that the mechanical problem is 

 precisely the same, whether the air be in motion and the bird at 

 rest, or the bird in motion and the air at rest. In each case the 

 relative motion is the same. 



Suppose, for example, a bird were poised at rest in a westerly 

 breeze, moving over the earth's surface at the rate of twelve miles 

 an hour, and suppose also the surface of the earth on account of 

 latitude to be moving at an equal rate in tlie opposite direction. 

 To anyone stationed on the surface of the earth this would be a 

 case of the bird remaining still in a moving current. Yet, in 

 fact, the bird would really be moving through still air at the same 

 rate as the surface of the earth. This, I think, will be sufficient 

 to illustrate the fact that the motionless poising of a bird in a 

 uniform current is identical with its uniform motton through still 

 air without change of attitude. 



I need hardly point out that the muscular effort necessary to 

 maintain the required attitude, producing no actual motion, can 

 do no mechanical work. It cannot overcome atmospheric friction, 

 nor the effect of the attraction of the earth. 



Perhaps, indeed, the following simple way of viewing the sub- 

 ject may render it still more obvious : — 



1. If the bird were deprived of its motor weight, i.e. if it were 

 exactly of the weight of the atmosphere, then, whatever might 

 be ifs motionless attitude, it would clearly float away like a balloon 

 with the atmospheric current in which it was immersed. 



2. If the air were at] rest, then also under the same circum- 

 stances it must necessarily fall towards the ground, either verti- 

 cally or obliquely, owing to its weight. 



3. Therefore, by the most elementary law of the composition 

 of motions, it follows that, taking into account the weight of the 

 bird and the motion of the atmosphere, the actual resultant 

 motion will be a motion combined of a motion vertically down- 

 wards and one or more horizontal motions. 



4. The resistance of the air on the relatively still wings of the 

 bird enables it to convert its downward motion partially into a 

 forward motion also ; but it is quite obvious that a motion com- 

 bined of horizontal motions and a downward motion must result 

 in a downward motion, and cannot produce equilibrium. 



The Duke of Argyll's testimony to the fact that birds hover 

 apparently without motion in horizontal air currents is valuable, 

 and no doubt increases the difficulty of accounting for the pheno- 

 menon on the hypothesis of upward currents. 



Graaf Reinet College F. Guthrie 



To Microscopists and Entomologists 

 Can any of your readers who are microscopists and entomo- 

 logists help me to a successful method of killing and mounting 

 Hoplophora deciimana — belonging to the order Acarina ? 



The difficulties it presents are, that on being touched it con- 

 tracts its head and legs and withdraws them into the horny 

 envelope which surrounds its body, and that portion of the 

 envelope extending over the head then closes tightly upon the 

 aperture, completely shutting in the head and legs, so that in 

 this condition the creature appears like a very minute seed 

 covered with a few spinous projections. I can find no certain 

 method of causing it to die unclosed, or so to mount it as to 

 exhibit its form ; and as the creature is not easily met with, I 

 shall feel much indebted by any suggestions. I may add that I 

 have consulted experienced mounters without success. 

 Hill Top, Midhurst, Feb. 22 R. C. Fisher 



* See Nature, vol. x. p. 262. 



