414 THE EMPIPvE OF THE AIR. 



in ii(» reef, but keeps an eye on his sails, is found to bloM' forty-four 

 miles an hour 1,500 feet up above the sea. The great Xorth wind, 

 measured by the transit of the cloud's shadow, blows from 67 to 89 

 miles per hour, while a violent "Kamsin" at a height of 500 yards shows 

 incredible speed. 



In this terrible wiiul, a tawny \'ulture moving with it, has a fright- 

 ful velocity; in a moment it has traversed the field of vision, say (for a 

 bird of that size) four or five miles. 



These are the tempestuous winds which expatriate the birds, which 

 cause the creatures, after a day's journey, to find themvselves 3,000 

 miles from their own habitat. 



These enormous velocities are j)roved by actual facts. The balloon 

 "Ville d'Orleans,'* which left Paris during the siege at 11 :4:5 p. M., 

 arrived next day near Lifjeld (Norway) at 3:40 p. m. Say 900 miles in 

 15 hours, or 60 miles an hour. The balloon launched at the coronation 

 of Xapoleon I, travelled during seven consecuti^•e hours at a speed of 

 about 90 miles an hour. During a long summer's day, say 18 hours, a 

 bird swept away by such a current of air, and rowing in the same 

 direction, might travers 1,800 miles! - - 



What splendid journeys a powerful wind might enable man to make 

 if he coitld navigate the air! But let us entertain no illusions; it will 

 be the accident, not the rule. - - - 



Let us now consider every day winds, those of moderate velocity. 

 Observation indicates, by comparing birds' progress with that of rail- 

 way trains, that the slow flyers go at most 155 miles an hour, and 

 that birds well endowed, such as the turtle-doves and the large sailing- 

 birds, in full flight through si^ace, get over some 37 miles in the hour. 

 So that, for general use, we may assume a speed of a little over a mile 

 in two minutes, as a probable achievement, if man sails on the wind. 



If the problem can be worked out, if the skill be acquired, this is the 

 rate of translation man may expect to compass, less perhaps, rather 

 than more. But it is a fair promise. He may journey 300 to 400 miles 

 in the day of 10 hours, with no expenditure of ])ower whatever, for the 

 wind will do the work. 



Velocity of the bird. — The speed of the bird's translation, considered 

 generally, especially for sailing birds, is comjiosed of the bird's ve- 

 locity with that of the wind. With flapping birds the case is difl'erent, 

 and the speed results from three factors; the speed of wind, the theo- 

 retical speed of the bird (which is to be estimated as if he were a 

 sailer), and the additional speed produced by persoiml exertion. This 

 latter speed, already object of many exjieriments within doors, and of 

 reams of calculations, nowise interests the obserAcr who watches the 

 sailing bird in all the simplicity of its flight. We therefore Avill con- 

 sider only the lesson to be learned from j)racticnl performances, 



