2;o 



NATURE 



{jfan. 3, 1889 



THE SOARING OF BIRDS} 

 CO much for sails. Now I want to make some sug- 

 ^ gestions, or suggest some queries, as to the skimming 

 flight of birds, in reference to which a good deal of fresh 

 observation has been possible during the voyage. 



You perhaps recollect that when the British Association 

 was at Glasgow, you asked me to put into writing, briefly, 

 as a paper for your Section, some remar'.cs on this subject 

 which I had made to you in conversation, but that, owing 

 to my hasty departure to attend the trial of H.M.S. Shah, 

 I omitted to do this. 



I had better briefly recite the above particulars here 

 in order to make more clear the bearing of the new 

 observations we (I and Tower) have made. 



The view was that when a bird skims or soars on 

 quiescent wings, without descending and without loss of 

 speed, the action must depend on the circumstance that 

 the bird had fallen in with, or selected a region where the 

 air was ascending with a sufficient speed. In still air the 

 bird, if at a sufficient height, could continue to travel 

 with a steady speed, using his extended wings as a sort 

 of descending inclined plane, the propelling force depend- 

 ing on the angle of the plane and on the equivalent of 

 " slip,"— that is to say, on the excess of the angle of actual 

 descent compared with the angle of the inclined plane. 

 The steady speed would be attained when the weight of 

 the bird and the sines of the angle of the plane = the 

 \i\x^''=, air resistance, including skin friction of wings — in 

 fact one might say := simply the skin friction of the whole 

 area, for the bird's lines are fine enough to justify this 

 statement, since there is no wave-making to be done, and 

 indeed experiment shows that the statement is true for 

 '' fish-formed" bodies moving wholly and deeply im- 

 mersed in water. Of course the bird's angle of actual 

 descent is greater than that of the quasi-inclined plane, 

 owing to the equivalent of " slip " in the wings. Under 

 these simultaneously acting and correlated conditions 

 there is of course— or probably— some total angle of 

 descent which enables the bird to minimize his rate of 

 approach to the earth in still air. If when there is a wind 

 the configuration of the ground or any other circumstances 

 can produce a local ascent of air more rapid than the 

 bird's minirnum rate of descent when soaring in still air, 

 he may continue to so.ir indefinitely by keeping in the 

 region where the air is thus ascending. 



Now, in most cases where one sees birds "soaring," it 

 is easy to see that they have plainly selected such a 

 region, and for a long time I felt confident that the only 

 two even apparent exceptions I had encountered were 

 such as to prove not to invalidate the rule. One of these 

 exceptions was that once, when the sea in Torbay was 

 in a state of glassy calm, I noticed a large gull thus'soar- 

 ing at sorne distance from the shore,— watching it with a 

 pair of binoculars, so that I was sure of the quiescence 

 of the wings. But here the riddle was at once solved by 

 the observation of what I had not at first noticed -the 

 dark trace of the front line of a fresh sea-breeze ad- 

 vancing all across the bay. Such an advance with a 

 definitely marked front, encountering an extended body 

 of quiescent air, involved of course an ascent of air in 

 the region of the encounter, and this was where the 

 bird was soaring. The other exception was that when 

 at sea I had often noticed birds thus soaring near the 

 ship. The solution was that, so far as I had then 

 noticed, the birds always selected a region to leeward of 

 the ship, where the eddies created by the rush of air past 

 her hull, &c., might readily have created local ascending 

 currents. 



The new exceptions we have seen since we have ap- 

 proached the Cape entirely set these two solutions at 

 defiance. 



' Extract from a letter of the late William Froude to Sir W. Thom-on, I 

 «if February s. 1878, received after Mr. Froude's death. Reptin'ed f.orn I 

 the ProceeJinijs of the Royal Society of Edinburgh, March 19, i883. ' 



The first exception we noticed was in the flight of some 

 albatrosses. We were sailing, and steaming (at low speed, 

 being short of coal), neariy du3 east in the latitude of the 

 Cape, with the wind light and variable abaft the beam, 

 and with a well-marked south-west swell of about 8 to 9 

 seconds period, and varying from 3 or 4 feet to 8 or 9 

 feet from hollow to crest. The speed of such waves would 

 be from 24 to 27 knots. 



Under these conditions the birds seemed to soar almost 

 ad libitum both in direction and in speed ; now starting 

 aloft with scarcely, if any, apparent loss of speed ; now 

 skimming along close to the water, with the tip of one or 

 other wing almost touching the surface for long distances, 

 indeed now and then actually touching it. The birds 

 were so large that the action could be clearly noted by 

 the naked eye even at considerable distances ; but we also 

 watched them telescopically, and assured ourselves of 

 the correctness of our observations. The action was the 

 more remarkable owing to the lightness of the wind, 

 which sometimes barely moved our sails, as we travelled 

 only 5 knots before it, by help of the screw. 



After long consideration the only explanation of at all 

 a rational kind which presented itself was the following, 

 which indeed presents the action of a vera causa, and 

 one which was very often certainly in accordance with 

 the birds' visible movements, though it was often also 

 impossible either to assert or to deny the accordance ; 

 and anyhow the question arises, Is the vera causa 

 sufficient ? I will try to trace its measure 



When a wave is say of 10 feet in height and say 10 se- 

 conds period (a case near enough to ours to form the basis 

 of a quantitative illustration) the length of the wave from 

 crest to crest is just 500 feet, the half of which space, or 250, 

 the wave of course traverses in 5 seconds, and assuming the 

 wave to be travelling in a calm, it must happen approxi- 

 mately that during the lapse of this 5 seconds the air which 

 at the commencement of the interval lay in the lowest part 

 of the trough has been lifted to the level of the crest, or 

 must have risen 10 feet, so that its mean speed of ascent 

 has been 2 feet per second (10 feet in 5 seconds). And 

 since (as is well known) the maximum speed of an 



harmonic motion is - times, or nearly \\ times its mean 



speed, it follows that along the side of the wave at its 

 mid-height the air must approximately be ascending at 

 the rate of 3 feet per second, and if the bird were so to 

 steer its course and regulate its speed as to conserve this 

 position he would have the advantage of a virtual upward 

 air current having tnat speed. 



NOTES. 

 The Berlin Academy of Sciences has presented 2000 marks 

 (;^ioo) to Prof. Leopold Auerbach (Breslau), and the same 

 amount to Dr. Franz Schiitt (Kiel), to aid them in their physio- 

 logical researches. Dr. Freudenthal, Professor of Philosophy at 

 Breslau, and Herr von Rebaur-Paschwitz, the astronomer, have 

 received 1500 marks (;^75) each. 



At the last meeting of the Scientific Committee of the Royal 

 Horticultural S cie'y, Mr. Henslow called attention to the fact 

 that the year 1889, besides being the centenary of the chry- 

 santhemum in Europe, is that of the dahlia in England. It 

 was introduced by the Marchioness of Bute in 1789, and figured 

 with single and double forms in the Botanical Magazine, vol. 

 xliv., t. 1885, and the Botanical Register, vol. i. t. 55. 



The death of Mr. J. J. Coleman, F.R.S.E., is announced. 

 He died at the age of fifty. For some time he was manager 

 of the works' of Young's Paraffin and Mineral Oil Company, 

 Glasgow ; and in this capacity he carried out some important 

 experimental investigations for the utilization of so-called waste 



