MIGRATION OF BlflDS. 259 



birds of passage ; for although they do not migrate to 

 distant regions, the greatest part of them make fre- 

 quent removals from one neighbouring district to an- 

 other, or from the interior of the country to the sea- 

 coast. The causes f these migrations, although en- 

 veloped in obscurity, appear, according to the most 

 probable conjectures, to arise from the failure of their 

 accustomed food, or the change of the seasons. 



The manner of performing the long flights, which 

 many of those birds take across immense tracts of wa- 

 ter before they arrive at any place of rest, throws for- 

 midable difficulties in the way of investigation; but 

 we ought, my dear Sir, to consider, that being accus- 

 tomed to measure distance, with relation to time, by 

 the speed of those animals, with which we are well 

 acquainted, we are apt to overlook the superior velo- 

 city of birds, and the ease with which they continue 

 their exertions. 



Our swiftest horses are supposed to go at the rate of 

 half a mile in somewhat less than one minute ; but 

 such a degree of exertion soon produces debility, and 

 cannot be long continued. With birds, the case is 

 very different; their motions are not impeded by si- 

 milar causes. They glide through the air with a ve- 

 locity superior to that of the fleetest quadruped, and 

 can for a great length of time continue their motion. 

 If we suppose a bird to proceed at the rate of no 

 more than a mile in two minutes for the space of 

 twenty-four hours, it will, in that time, have passed 

 over an extent of more than seven hundred miles; 

 and, if aided by a favourable current of air, there is 

 reason to suppose that the same may be performed in 

 a much shorter space of time. 



If it be asked how they know the time when to 

 commence their migrations, and in what manner to 

 direct their course, it may with great propriety be 

 answered, that the same All-ruling Power which be- 

 stowed reason on us has given instinct to them : the 

 changes in the atmosphere may indicate the proper 

 time of removal; and it is also to be observed, that 

 their course is determined rather by the weather. 



