Manchester Memoirs, Vol. xliv. (1H99), No. 5. 3 



component may thus maintain relatively to the ground an 

 absolutely fixed position, or he may advance over the 

 ground to windward at a fixed level. 



There can be no doubt that the vertical component of 

 wind i)la)'s a large [iart, not merely in the (light (;f birds, 

 but in general atmospheric phenomena. Living at the 

 bottom of the atmospheric ocean, where the wind is 

 necessarily parallel to the ground, we are liable to overlook 

 the importance of vertical motions. This is the more 

 remarkable when we consider that wind is due to atmos- 

 pheric expansion and condensation, so that the primary 

 movements are vertical and not horizontal. Thus the 

 inhabitants of an oceanic island are specially interested 

 in the so called land and sea breezes, but the primary 

 phenomenon is the rise and fall of air over the island as 

 it is heated by the sun during the day and cooled by 

 radiation at night. 



A recent American observer (Iluffaker, Smithsonian 

 Report for 1897) has recorded many examples of vultures 

 soaring under circumstances which suggested that they 

 take advantage of the upward currents which rise locally 

 from the ground when it is strongly heated by the sun. On 

 dull days and in light winds the vultures were not seen to 

 soar. There is no doubt that under the influence of a 

 strong sun the layers of air near the ground approach an 

 unstable condition, and that comparatively slight causes 

 may determine local upward currents. Mr. Huffaker 

 suggests that in some cases the birds themselves, by flying 

 round, may determine the upward current. Some of his 

 observations certainly point in this direction ; but it must 

 be remembered that the immediate effect of flight will be 

 a downward and not an upward current. 



The more obvious examples of upward motion occur 

 when an otherwise horizontal wind meets an obstruction. 



