THE VICTORIAN NATURALIST. 15 



Mr. S. E. Peal. It rests on the fact that the velocity of the wind 

 always increases with the height above the ground, owing to the 

 lower strata being retarded by friction. Now in ascending 

 against such a wind, a bird will be meeting stronger currents in 

 its face as it rises — that is, it will be gaining in relative velocity, 

 and in descending with the wind they are also gaining in relative 

 velocity, since the lower strata really have a velocity, as compared 

 with the upper strata, in the opposite direction. A bird may 

 circle around indefinitely on this principle, the circles being 

 inclined downwards to leeward, provided the successive gains in 

 relative velocity balance the resistances to its motion. The 

 ascent against the wind is made with inclined body, so that the 

 pressure underneath is greater than the weight of the bird ; but 

 the atmospheric resistance is also great, and the ascent is made 

 as quickly as possible. On wheeling around to descend with the 

 wind the relative velocity will be small and the lifting pressure 

 less than the weight of the bird. In falling into the slowly 

 moving strata it will be really meeting a stronger wind in its face, 

 having itself a velocity at least equal to that of the upper stratum, 

 and will also be aided by the upward pressure due to its vertical 

 descent. We possess no experiments to determine this latter 

 pressure, but it is probably greater than if the bird were falling 

 with the same velocity in still air. An albatross does not, 

 however, move always in circles ; it seems to make best progress 

 in a direction across the wind, alternately rising quickly against 

 and across the wind and then gliding downwards with the wind 

 and across in the same direction as before, the resultant motion 

 being at right angles to the wind. It may be asked what 

 evidence is there that this variation in the velocity of the wind 

 actually exists. In 1889, at the Eiffel Tower, in Paris, experi- 

 ments were made upon the relation between the velocity of the 

 wind at the top and at a station 70 feet from the ground. It was 

 found that the velocity at the top was from two to five times as 

 great as at the lower station. In Nature, 22nd April, 1886, Mr. 

 E. D. Archibald records observations made with kite-wire sus- 

 pended anemometers, which show that the velocity in high winds 

 was 38 per cent, greater at 250 feet above the ground than at 100 

 feet above, and 56 per cent, greater at 550 feet up, and also that 

 there is a steadily diminishing increase up to a height of 1,300 

 feet. It has been contended that the progressive increase in the 

 velocity of the wind does not extend sufficiently above the earth 

 to account for the soaring of eagles at great heights, and that they 

 may gain in relative velocity by always facing gusts ; but the 

 regularity of their movements seems to preclude this view, and it 

 must be remembered that these birds take very large sweeps, and 

 are much lighter in proportion to their supporting area than an 

 albatross, and can take advantage of smaller differences in velocity. 

 The difference between the velocity of the wind in the troughs of 



