I48 THE REIGN OF LAW. 



The contrast between the violent efforts required 

 for the first acquisition of the initial velocity, and 

 the perfect ease with which flight is performed after 

 it has been acquired, is a contrast described by 

 Virgil in lines of incomparable beauty : — 



" Qualis spelunca subito commota columba 

 Cui domus et dulces latebroso in pumice nidi, 

 Fertur in arva volans, plausumque exterrita pennis 

 Dat tecto ingentem ; mox, aere lapsa quieto, 

 Radit iter liquidum, celeres neque commovet alas." 



— ALn. lib. v. 213-17. 



Still more remarkable, as showing the power and 

 the value of weight in flight, is the fact that birds 

 are able to resume rapid and easy motion not 

 only as the result of a previously - acquired 

 momentum, but after " soaring " in an almost 

 perfectly stationary position. Nothing, for ex- 

 ample, is more common than to see Sea Gulls, 

 and some large species of Hawks, "soaring" one 

 moment, (that is, all the forces bearing on the 

 bird brought to an equilibrium, and all motion 

 brought consequently to nearly a perfect stand- 

 still,) and the next moment sailing onwards in 

 rapid and apparently effortless progression. Now, 

 how is this effect produced ? If we only think of 



