132 COMPARATIVE SWIFTNESS 



that^ for the distance they go, they run much faster 

 than a hare ; a fact clearly shown by this circumstance, 

 that few greyhounds ever kill a rabbit in coursing : 

 when they happen to catch them, it is by surprise, or 

 a sudden spring. The slowest flying hawk will strike 

 upon a rabbit with scarcely an appearance of moving ; 

 so much more rapidly does it fly, than a rabbit can 

 run. Yet this hawk flies too slowly to catch a par- 

 tridge : a much swifter species of hawk is requisite 

 for that pursuit, and one of still greater speed for a 

 pigeon; but for a swallow, a hawk must be found 

 that can outstrip all these competitors. Therefore, if 

 we take the speed of the swiftest horse at sixty miles 

 an hour, (though no horse ever maintained that pace 

 for so long a time,) as a standard by which the rest 

 may be estimated, and pursue the calculation from the 

 horse to the greyhound, hare, rabbit, slow hawk, par- 

 tridge, pigeon, swallow, and hawks of the swiftest 

 kind, we may pretty satisfactorily account for the 

 amazing rapidity with which birds perform long voy- 

 ages. If we only suppose them to go at the rate of 

 half a mile in a minute for the space of twenty- 

 four hours, they will pass over in that time an extent 

 of more than seven hundred miles ; but, if assisted by 

 a favourable current of air, there is reason to sup- 

 pose that an equal voyage may be performed in less 

 time." 



