THE OTAGO INSTITUTE.

Otago Daily Times, Issue 2436, 26 November 1869, Page 7

THE OTAGO INSTITUTE.

At the recent meeting of this Society Mr J. S. Webb ..read a paper "On the mechanical principles involved im the sailing flight of-the Albatross," of Which the following is an abstract :— \

In the first volume of the Transactions of the Newr Zealand Institute there is an abstract pf;a,paper:by Captain Hutton on this subject.; ; EorYjvarit!'-„pf.'. necessary. type for the algebraic formulae the ! paper could not be printed in fuILY A J revised and modified copy of the more important portions of it has ho wever been published in the Philosbphical Magazine for August last. The subject 4ealt with by Captain Hutton is what has been somewhat inappropriately called the 11. sailing flight" of the albatross—that steady ahd continued motion, without any observable action of the wings, which-has attracted"the attention of every one who lias made a voyage on the Southern Seas. The paper is ably written and very interesting, but in his mathematical treatment of the subject Captain Hutton ha.s not been happy, having, as it,appears to me, made a mistake at the outset of his calculations, The object of the present communication is to supply what I consider to be the! necessary corrections, and to show the effect which the new results thus arrived at have upon the general conclusions dr!awn by Captain Hutton frorii his own calculations. -Asi cannot ask this meeting to follow me through a dry, though by no means abstruse, process of mathematical reasoning, I have thrown that portion of the paper into tbe form of an appendix, which I lay on the table for perusal by those members who may te desirous of examining it.

In his paper, Philosophical Magazine, August, 1869, p. 130, Captain Hutton proposes to himself "to determine approximately the probable resistance of the air in order to . allow the albatross to sail for ..half an hour without moving its wings." He estimates the under surface of the wings, body, and tail of the bird at 8 square feet, and the weight of the bird at 161bs. From these data he concludes that "if an albatross starts with a velocity of 115 feet per second, it could maintain a constant height above the sea until its velocity was reduced to 64 feet per .second, by merely increasing the angle to the horizon at which it was flying from Odeg. to 7deg." He assumes that the wings are always inclined lodeg. more than the body of the bird. The corrections 1 offer today show that, on the data assumed by Captain Hutton, the velocity at starting must be 155 feet per second, instead of 115, and that it must not fall during the interval below 100 feet per second, instead of to 64 feet.

A consideration of the following extract from Captain Hutton's paper will show the hearing of this correction on his general results. " The velocity of the air in a ' fresh sailing breeze' is about 30

feet per second ; in a 'moderate gale,'.6o feet per second ; in a 'strong gale,' 00 feot por second ; and in a ' great uturm,' 120 feet per second. Now, an albatross can. often/be seen sailing, though slowly, directly against a strong gale, liis volocity (through the air) must therefore often be more than 90 feet per second ; he is, however, most at home in a stroug breeze or moderate gale, when the velocity of tho wind ia 50 or 60 feet per second, and consequently when his velocity would have to be 70 or ,80. feet per second to onable him to fly easily against it. In a calm or light air, when the; wind has a velocity of only 10 feet a second, the albatross rarely sails for so> long as a minute at a time, the reason for this being that as, in order to sustain himself in,therair, he must move through it with a -velocity not less', than 64 feet per second, he would, even when flying.against the wind, have to travel over the sea at the rate of not less than 64 feet per second, or 36 miles an hour, and so could not roach ft properly for good, nor stop himself quickly enough when he saw anything ; so that the velocity and manner of flight observed in the albatross correspond closely enough with those calculated as necessary from theoreticalrconsideratioris." It will readily be seen, that the very Imuch .higher velocities I derive ) from "Captain .Hutton's ; data, upset the* conclusibnhe here draws. ; It is, however, iri the choice of his assumed data, and in his calculations based; upon therh; and not in the principle V by wliich he accounts for the power of -the albatross to.sriirfpra long time :>yithput moving ■•itsi. wiriga, that CJaptain l: Hritton is in '&rror. I do not kribw-;tohetheYr the .'{merit of, the demonstration belongs to himr (he; appears to claim it)'■; but,! if- so,'.notwithstanding the criticisms . I have ventured upon, I willingly >bear testimony to his success ih ' the "primary object of his paper, viz. "to indicate the principles involved in the flight of the albatross when sailing along without moving its wirigs." Captain Huttori 'proceeds to calculate from his first results, "what the; resistance of the air to the forward progress of the albatross 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,vand at the end of that time to have rednced'his velocity to 64 feet per second:"V He..arrives at a result for wliich he himself finds it necessary to offer excuses, viz., that the resistance to a body of the shape of this bird .is only 1-300 th of that to round shot. Had he used the figures which 1 have brought out instead of his own, his 5, 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 aro available by the aid of the undoubtedly true principle laid down by Captain Hu ton. I repeat an,d endorse his own closing remark, "The problem still remains to be solved ; but, until experiments have been made on the resistance offered to tlie air by the front and lower surfaces of birds, a tolerably accurate solution is not possible ;" and I may add, that sorrie careful observations of the duration of the "sail- ; ing flight" of various birds, and of their ordinary position in the air whilst flying without flapping the wingS, are abdoluteiy necessary.before ; anything liks-approxi-mately correct calculations on the subject can be made.

The remainder of this paper is entirely occupied by mathematical reasoning and calculations, and concludes with the suggestion of an entirely different method of treating the subject mathematically from that adopted by Captain Hutton.