THF COMMON SWIFT. 251 



lower down ; and therefore more delicate but not less certain 

 indications of changes might be drawn from them. 



It appears from their mode of flying, that in very dry and 

 settled weather, there are fewer insects at a great height in 

 the atmosphere than when the air is troubled, even when the 

 signs of that trouble have not shown themselves in the depo- 

 sition of vapour, or in the weight of the air as shown by the 

 barometric column. It may be (and it most likely is) that 

 the evaporative power of the atmosphere is the first property 

 of it that yields ; and that their weak wings, which could not 

 bear the drying influence of the rarefied upper air when dry, 

 can endure it much better when its ardour relents. 



In dry weather, the swifts hawk only towards morning and 

 evening, flying lower down than when the air is different, and 

 occasionally skimming the surface of the pools, and sipping and 

 laving themselves as they dash along. At these times, too, 

 they are sportless and silent, and if the drought is of long 

 continuance, they seem fatigued ; but when the upper air 

 relents they fly high, appear all day on the wing, accumulate 

 in unwonted numbers, gliding, dashing, wheeling, playing 

 numerous antics, screeching to each other, and apparently 

 acquiring more energy the longer they are on the wing. 

 These sportive dashings in the upper air become more nume- 

 rous and energetic as the time of their departure approaches, 

 as then their care of their broods has ceased, and they have 

 only their own food to find each for itself. The solstitial 

 showers generally give them a farewell feast ; and at that time 

 they may be seen on the wing for sixteen hours in the day 

 without once alighting to rest. Their sight has, by experi- 

 ment, been found to be so very acute, that from a distance of 

 400 feet they can discern an object not more than half an 

 inch in diameter, and how much less than that is not known. 

 The same motive of exertion which they often perform in this 

 country without any apparent rest, would sufiice to carry 



