\.,I XXX 

 I'll 3 



Fobbi Concerning l/u Flight of Gull 36] 



could have been thus constantly and generally maintained by 

 other influences when the ocean all about her \\;i iwept by ;< wind 

 blowing over thirty-five mil< an hour. I even doubt if tin 

 tended much above her upper deck For there I wa lashed inces- 

 ;niil in the face by what eemed to be horizontally-racing wind, 

 while several of the gulls were often ailing fifteen or twent; feet 

 higher still, perhaps directlj over me. On the other hand l1 must 

 be admitted that I have never known any of these birds to glide 

 far to windward excepl when accompanying a iteamship, ■■> fact 

 which apparently lends ome lupport to Mr. Tydeman's conten- 

 tion, although not neces arilj having tuch ignificance ince it may 

 1 1. 1 onably \><- interpreted in other wa 



Willi the data to thoroughly determined the problem resolves 

 n elf into one of comparatively elementary ph., ic I have not 

 een Tydeman's article, but I believe that without In ab tru e 

 calculat ion it is po ible l<; rea oning which i . wit hin t he reach of 

 those untrained in higher mathematics to how thai the phenome- 

 non of horizontal gliding could not be produced in 'Ik- way Mr. 

 Brewster sugge I 



Let ii-. assume for the pre en1 that the wind is blowing horizon- 

 tally and uniformly, that the air i an evenly moving ma without 

 local di tortion . 'Jinn to ;< bird surrounded by the moving mass 

 it is as if the air were till and the earth's surface travelling bj 

 underneath. Tli<- bird will tend to drift freely with the wind and 

 will feel no more pr< ure from it than from the surrounding ;n'r 

 in ;i flat calm. The case of •> bird in contact with only one medium 

 moving uniformly i wholly different from that of a boat in contact 

 with two media, air and water. The tendency of the watei to 

 prevent free drifting with the wind i the ole cau le of the wind'-, 

 pn ure on the boat as long as the air moves uniformly. The 

 bird '"in only feel pressure from the urrounding air in ca e of a 

 sudden alteration in the peed or direction of the wind, or in ca e 

 of motion through the air imparted by v-i' :r - itj or bj the bird 

 effoi I 



Tin- problem of gliding can be I I"- con lidered by regarding the 

 bird as in a calm and anal) ling it po ible motion, with re ped 

 to the surrounding air regardless of tin- earth beneath. After this 

 anal; i we maj introduce the relative motion between air and 



