INFLUENCE OF WEATHER ON FLIGHT. 145 



swarm was passing over. He states ' that " they could be seen filling 

 the air like snow-flakes to a great height above the extreme summit of 

 the peak, 13,333 feet." 



It is evident from these facts that they can fly at an elevation of 

 15,000 feet above the sea-level, or about two miles and a half above the 

 general surface of Kansas and Nebraska j and far out of sight of the 

 keenest vision. 



As they are natives of the dry, elevated table-lands of the Northwest 

 and the region of rarefied air, it is more than probable that one object in 

 leaving the abundant pasturage they find amid the fields and prairies 

 of the border States is to search for a drier and more rarefied atmos- 

 phere; and led by instinct to seek for this in the upper regions, they con- 

 tinue to ascend until they reach the limit of their power of upward 

 flight. 



If this be true it will explain their often sudden and mysterious ap- 

 pearance at points far within the bounds of the settled areas without 

 anything having been seen of them along tbe line they came. It will 

 fully and sufficiently account for the difficulty of tracing individual 

 swarms along the line of their march. 



Effect of a change of wind or weather on flights. — A large number of 

 facts have been collected which go to prove that as a very general, or, as 

 we may perhaps truly say, universal rule, a sudden change of wind brings 

 a flying swarm to the ground. From the fact that very often with a re- 

 turn of the wind to its former direction tbey rise and move on in the 

 course they were going when stopped, it has been taken for granted 

 that they have a desire to move in a given direction, and will therefore 

 only rise and fly when the wind is blowing in a direction that will enable 

 them to carry out this purpose. The very large number of instances 

 noted where they have been stopped by contrary winds and refused, 

 apparently with a dogged determination, to move until the wind was 

 right to carry them on in their original course, would seem to be suffi- 

 cient to confirm this theory. That it has, at least, some foundation in 

 fact we think must be admitted; yet we are satisfied it is used to ex- 

 plain numerous movements and phenomena which are due to other and 

 wholly difl'erent causes. 



That a sudden change in temperature to a lower degree will as surely 

 bring a flying swarm to the ground as a change of wind, is now" admitted 

 by all who have studied or observed the habits and movements of these 

 insects. This is probably one great reason why they so generally come 

 down at the approach of night, as the temperature usually falls very 

 rapidly on these Western plains as the sun disappears. Whether they 

 ever wheel round in the air and move back on their course without first 

 coming down it is impossible to say, but so far no case of the kind has 

 been observed of which we have received afiy account. Some of the 

 movements in Minnesota during the past year led Mr. Whitman to think 

 such a thing possible, as it would enable him to explain certain facts 

 10 G 



