234 



considerations." It will readily be seen that the Very much higher velocities 

 which I derive from Captain Hutton's data, upset the conclusion he has here 

 di'awn. It is however in the choice of his assumed data and in his calculations 

 based on them, and not on the principle by which he accounts for the power 

 of the albatros to sail .for a long time without moving its wings, that Captain 

 Hutton is in error. I do not know whether the merit of the demonstration 

 belongs to him (he appears to claim it), but if so, notwithstanding the criti- 

 cisms I have ventured upon, I willingly bear testimony to his success in the 

 primary object of his paper, viz., to " indicate the principles involved in the 

 flight of the albatros when sailing along without moving its wings." 



Captain Hutton proceeds to calculate from his first results "what the 

 resistance of the air to the forward progress of the albatros ought to be, to 

 enable him to start with a velocity of 115 feet per second, and sail for half 

 an hour without flapping his wings, and at the end of that time to have 

 reduced his velocity to 61 feet per second." He arrives at a result for which 

 he himself deems it necessary to offer excuses, viz., that the resistance to a 

 body of the shape of this bird is only l-300th of that to round shot. Had 

 he used the figures which I have brought out, instead of his own, his estimate 

 would have been only about half what it is, — a further proof, if any were 

 needed, that the real details of the bird's flight are very different to those 

 assumed in his calculations. I have not the necessary leisure to attempt to 

 deduce these details from such physical data as are available by the aid of 

 the undoubtedly true principle laid down by Captain Hutton. I repeat and 

 endorse his own closing remark: — "the problem still remains to be solved; 

 but until experiments have been made on the resistance offered to the air by 

 the front and lower surfaces of birds, a tolerably accurate solution is not 

 possible." I may add, that some careful observations of the duration of the 

 " sailing flight " of various birds, and of their ordinary position in the air, 

 whilst flying without flapping the wings, are absolutely necessary before any- 

 thing like approximately correct calculations on the subject can be made. 



APPENDIX. 



The references in what follows are to the annexed copy of Captain Hut- 

 ton's diagram, to w T hich I have added the arc H A' C, and the dotted lines 

 A' T and C S. 



Captain Hutton assumes the under surface of the bird at 8 feet, its 

 weight at 16 lbs., the surface of the wings at (about) three times that of the 



body and tail, and the upward current of 

 air necessary to support the bird against 

 gravity, at 30 feet per second acting upon 

 the whole bearing surface. 



" Let A B " he says, " represent the 

 axis of the body of the bird flying in 

 the direction B A and at an angle A E H 

 to the horizon. Let C D represent the 

 wings making Z C E H with the horizon. 

 Take the line H E to represent the ve- 

 locity at which the bird is flying, or the 

 number of feet it passes through the air 

 in one second. From H draw the perpendicular H A, the line will repre- 

 sent the distance which the bird will rise (omitting for the present the force 

 of gravity) by means of the angle at which he is flying to the horkon." 

 Here Captain Hutton first assumes that the number of feet the bird travels 

 in one second =HE and then that the bird will pass in the same time 

 through the longer distance A E. The mistake leads him to the further error 

 of adopting H E tan A E H as the measure of the vertical component of the 



