Feb. 24, 1876] 



NATURE 



325 



satisfactory velocity. Its subsequent sailing flight until it again 

 increases its rate of speed by flapping, I would suggest to be 

 merely a utilisation of this original rwzTt a to the utmost possible 

 advantage, the ascending and descending movements of the bird 

 being nothing more than a change from actual to potential energy, 

 and rice z'crsA. Suppose, for the sake of simplicity, that the wind 

 is dead ahead, and that the bird commences sailing horizontally with 

 a certain vis vita. With this, by fixing its wings so as to present 

 inclined planes to the direction of the wind, it is able to rise 

 to a certain height, the velocity decreasing in some ratio to the 

 ascent, and if the highest point capable of being reached is 

 attained, the bird for the instant comes to rest; up to this 

 moment the actual energy has been gradually changing into 

 potential, and the bird gaining thereby a position of advantage. 

 It is, however, extremely rare that this position is attained — 

 most frequently the horizontal velocity is only partially destroyed. 

 The planes of the wings being now changed with reference to 

 the direction of the wind, the bird begins to descend ; the 

 potential energy is transformed into actual, and velocity is 

 acijuired, to be again changed into potential; and so on until it 

 becomes necessary to renew it. The hne of flight, therefore, of 

 an albatross going directly against the wind consists of a series 

 of undulations, the summits of which correspond to the instants 

 of least relative velocities, or positions of greatest potential 

 advantage ; whilst the lowest points correspond to the instants 

 of greatest relative velocity and least potential advantage. 



During all this time vis viva is of course being extracted by 

 the resis:ance of the wind, and the velociiy after a while is so 

 diminished that the bird loses its power of rising to a satisfac- 

 tory position of advantage. It is then that flapping rec:m- 

 mences and new power of flight is acquired. When it is re- 

 membered that the weight of a Cape albatross varies from 

 16 lb. to 20 lb., and the stretch of wings from 10 to 12 feet, it 

 will be evident how great is the potential energy of such a bird 

 at the height say of loo feet, and also how complete is its power 

 of utilising that energy. The question may be asked, how long 

 will it be before 2,000 foot-pounds of work have been extracted 

 by air movmg at the rate of sixty miles an hour ? for until it has 

 been extracted, or nearly so, the sailing flight of the albatross 

 need not cease. By means of a suitable mechanism for changing 

 the inclination of the wing planes every few seconds, the sailing 

 of the albatross, I believe, might be simulated without great 

 difficulty. It is generally supposed that the stronger the wind 

 the greater is the power of sailing-flight. In the special instance 

 referred to, viz. , that of sailing directly in the teetn of the wind, 

 this is not the case. A good breeze is evidently better than 

 either a very strong wind or a calm. In the one case, a too great 

 resistance destroys the vis viva too rapidly ; in the othe r, the 

 bird suffers from a want of sufficient resistance, very much as a 

 kite does during a calm. 



In sailing in any other direction a violent wind may more or 

 less aid the flight, and the velocity attained in some instances be 

 enormous and very deceptive. It is this element, viz., the velocity 

 acquired by sailing obliquely with the wind, that is so difficult 

 for the eye to eliminate in estimating the actual power of the bird 

 to sail against the wind. In flying with the wind, the resisiance 

 to the stroke being greater, the necessary speed may be more 

 rapidly acquired and with fewer strokes, provided the bird has 

 the requisite strength. But, as might naturally be supposed, sail- 

 ing directly with the wind for any considerable distance is rarely 

 or never seen, the bird not finding sufficient resistance in the air 

 for its support. 



From what has been said it will appear that the superior sailing 

 power of the albatross, in comparison with other birds, is due — 



(1) To its ability to acquire readily very great vis viva by 

 means of its extremely powerful wings. 



(2) To its almost perfect power of utilising this vis viva for 

 the purpose of ascending or descending, i.e. of changing from a 

 position of greatest actual to greatest potential energy and vice 

 versa, with hast loss of power through resistance of the air. 



The above implies an extraordinary rigidity as well as absence of 

 concavity of the wings, by which the bird is enabled to hold them in 

 their place like two rigid planes, and thereby present their surface 

 to the wind under the most favourable circumstances possible. The 

 tremulous movement seen at the tips appears to be nothing more 

 than vibrations due to the want of absolute rigidity in the pinions. 

 The above suggestions, if tenable, furnish an explanation also of 

 the flight of the flying fish — the undulatory motion, or rising 

 over the crest of a wave, which has puzzled so many casual 

 observers, being merely a change of some of the vis viva of its 

 flight into potential energy. This means necessarily a loss of 



velocity depending on the amount of rise, and implies the power 

 of the fish to change its wing planes so as to ascend or descend. 

 The original vis viva has of course been created by a preliminary 

 rush through the water before emerging. 



It will be seen from what has been said that the principle 

 suggested, rightly orwTongly, as fully explaining the flight of the 

 albatross, is that of a body — gifted with the most perfect power 

 of placing itself in a position of advantage — sliding up and down 

 inclined planes under the most perfect conditions possible. 



R. A. 



The Use of the Words " Weight " and " Mass " 



In the review of Dr. Guthrie's " Electricity and Magnetism " 

 (Nature, vol. xiii. p. 263) the following words occur in refer- 

 ence to Dr. Guthrie's definition of the absolute unit of electric 

 resistance : "Here, irrespective of other considerations, there is 

 the fundamental error of using the term weight instead of mass." 



It is very unfortunate that the word " weight " is ambiguous ; 

 and that the ambiguity is actually so great as to lead to all but 

 universal confusion of ideas. It is not really improper to use 

 'weight as synonymous with fnass, and, had Dr. Guthrie meant 

 to refer to mass, his using the term weight would not have con- 

 stituted any fundamental error. He would only have been using 

 an old ambiguous word in the more authoritatively established 

 of its two common meanings. By an Act of Parliament (i8th 

 and 19th of Victoria, Chapter 72, July 30, 1855) for the special 

 purpose of establishing standard weights and measures, it is 

 enacted that a certain piece of platinum referred to as a " weight 

 of platinum " shall be denominated the Imperial Standard Pound 

 AvoirdufKjis, and shall be deemed to be the only standard of 

 weight from which all other weights and other measures having 

 reference to weight shall be derived, computed, and ascertained. 

 The gravity of a mass, or of a piece of matter, is not once named, 

 or in any way referred to, in the Act as a thing for which a 

 standard is meant to be established by that Act, nor is the word 

 force or the notion of force put forward in any way in the Act. 

 Thus the meaning attributed in the Act to the word lunght is 

 the same as is distinctly expressed in scientific language by 

 mass. 



However, on turning to Dr. Guthrie's book itself, I found 

 a striking example of the troublesome perplexity which is in- 

 volved in the ambiguity of the language in common use. 

 A few lines below the passage touched on by the reviewer, 

 the following sentence and appended note occur. (Text), 

 "From the work done by the current in the experimental 

 wire, the resistance in that wire is found, and this resist- 

 ance is considered unity when the above measures are units, 

 namely, I second time, I meter space, and i gram weight or 

 force.' (Xote appended), " The force actually taken as imity is 



g'^™* fo'" t^^ force acting on i gram for i second will 

 9'oi 



give it a velocity of i meter a second." The text and the note 



are utterly irreconcilable. The confusion is complete. 



1 do not say that no one can possibly understand the subject 

 with the common nomenclature ; but I do say, from considerable 

 experience in Glasgow University, where we are in the habit of 

 using the absolute or kinetic system of force-measurement in all 

 our calculations with the students of the Natural Philosophy 

 Class, that it is extremely difficult to explain, with the old nomen- 

 clature, the beautiful, and in itself simple, kinetic system of 

 Gauss, together with its connection with the gravitation system 

 of force-measurement. 



This session, however, I have found a very great simplification 

 in adopting a suggestion of Prof. James Thomson to do away 

 with the word weight altogether in cases in which its employment 

 would involve ambiguity. He would still readily use the 

 name, a pound weight, for the standard piece of iron or br^tss 

 used in weighing ; and would continue, so long as our present 

 non-decimal system is maintained, to use the commercial term, 

 a hundredweight of iron, meaning a certain quanuty or mass of 

 iron. But he has proposed that when we mean mass we should 

 avoid the word weight as far as possible and use the word mass, 

 and that where we mean downward force due to gravitation, 

 called by Dr. Guthrie and his re\'iewer, weight, we should use the 

 word gravity. Thus we may speak of a one pound force, or 

 we may say "the gravity of a pound," but never " the weight 

 of a poimd." We can scarcely get rid altogether of connecting 

 the idea of heaviness with the word weight, nor would our dic- 

 tionaries at present allow us to do so ; but it is quite proper to 

 feel that, in speaking of a certain weight as being too great t© 



