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NATURE 



[October i, 1891 



A Tortoise inclosed in Ice. 



During the last winter there was a good deal of correspond- 

 ence in the columns of Nature regarding the revivability of 

 fish and insects that had been frozen hard. A similar pheno- 

 menon with regard to the tortoise having recently come under 

 my notice, it may perhaps be interesting to some of your readers 

 to have it put on record. 



Some friends of mine have one of the small water-tortoises 

 that are occasionally exposed for sale in the City. Last winter, 

 this tortoise was inadvertently left in his small pond, the water 

 of which froze completely into one block of ice, inclosing the 

 tortoise. When the thaw came, the creature was found alive 

 and flourishing. I especially endeavoured to ascertain whether 

 the tortoise had been absolutely and completely inclosed in his 

 icy casing, or whether he had been simply frozen into the ice, 

 but partly inclosed and partly free. Unfortunately, however, 

 in spite of cross-examining several of the family, I was unable 

 to obtain a perfectly clear and definite statement on this point : 

 one of my friends, however, declared that, if not completely 

 encased, at any rate only the arch of the tortoise's back was 

 free. This is, however, sufficiently indefinite to debar one from 

 asserting that all access of air was denied to the tortoise ; and 

 that is the point on which my interest chiefly centred. 



F. H. Perry Coste. 



7 Fowkes Buildings, Great Tower Street, E.C., 

 September 25. 



The Soaring of Birds. 



I HAVE read with much interest Mr. Peal's account of the soar- 

 ing of vultures, pelicans, adjutants, &c., over the plain of Upper 

 Assam (Nature, May 21, p. 56). Their manner of flight is 

 identical with that of seagulls and harriers over the Canterbury 

 Plains in New Zealand, which is about 150 miles long and 45 

 wide in its widest part. These birds begin to soar at a height of 

 about 200 feet, and rise in slanting spirals to 2000 feet and 

 under. The gulls are much the most numerous, and flocks of 

 them may be seen soaring nearly every fine day in summer. 

 Sometimes a number assemble, and after going round in circles 

 for a short time, without rising, or rising very little, they come 

 down, the condition of the air being apparently unfavourable for 

 soaring. Whenever I have seen a flock finish an ascent, they all 

 reached the same height, which is consistent with the supposition 

 that they go as high as they can. They never remained at the 

 limit of their ascent even for a short time, but separated, sailing 

 away downward to great distances. 



The explanation of soaring given by Mr. Peal'can hardly be 

 the true one. Bishop Courtenay has shown its inadequacy by 

 proving that a bird in a uniform horizontal current is in no 

 respect more able to support himself than in a calm. Though 

 carefully looking for it, I have never been able to see the 

 descent which Mr. Peal supposes to be made (he does not say 

 that he has seen it) whfen the bird is going with the wind. 



The soaring of birds shows plainly that the velocity of the 

 wind over a flat country does not increase with the height in a 

 perceptible degree up to great heights. If there were such an 

 increase at anything like the rate near the ground, a bird soaring 

 would be out of sight long before he could reach loco feet, but 

 birds seem to drift horizontally at nearly the same speed during 

 the whole of their ascent. The increase of the velocity of the 

 wind with the height may be studied by observing the behaviour 

 of smoke or steam carried along : near the ground the increase 

 is easily seen, over 20 feet it is very small, over 50 seldom per- 

 ceptible, a wreath of smoke over that height being carried along 

 without any relative motion of the parts, or so little that it could 

 be of no use in soaring. 



In a description of the sailing flight of the albatross (Nature, 

 vol. xl., p. 9) I mentioned that when the wind is at right 

 angles to the course of a steamer attended by a flock of alba- 

 trosses, some of them occasionally follow the vessel not far astern 

 in undulating lines, rising against the wind and falling with it, 

 and turning alternately right and left ; also that seagulls do an 

 imperfect imitation of this kind of flight over flat country, nearly 

 touching the ground at each descent, as the albatrosses nearly 

 touch the sea. The gulls are evidently unable to reach the 

 height from which the previous descent was made without 

 flapping their wings a few times during the second half of each 

 ascent. Without doing this, they would soon come to the 

 ground, though using the differential motion of the air, where it 



NO. I 144, VOL. 44] 



is at its maximum, to the greatest advantage possible. It seems, 

 therefore, that soaring at great heights cannot be explained on 

 the same principle as the sailing flight of the albatross, whose 

 movements are confined to a comparatively thin stratum of air 

 next the sea, in which the velocity of the wind increases rapidly 

 with the height. 



In Lyttelton Harbour, N.Z., which is surrounded by hills 

 except at the entrance, the gulls soar by using the upward 

 current on the slopes, rising in spirals in precisely the same 

 manner as when soaring hundreds of feet above the plain. The 

 motive power in the former kind of flight is evident, and perhaps 

 throws light on that of the latter. Standing on a slope of about 

 20°, and about 100 feet above the sea, I saw a flock of gulls 

 sitting on the water. A breeze sprang up , and the whole flock 

 began to ascend over the slope. Being constantly among the 

 shipping they are very tame, and several came within 12 feet 

 of me. When moving against the wind their motion with 

 respect to the earth was very slow, so that I had a good oppor- 

 tunity of seeing if there was any vibratory movement of the 

 wings, but no movement of any kind was visible. The ascent 

 of birds over a slope by means of the current flowing up it, and 

 their descent in long inclines at a small angle with the horizontal, 

 show that rapid motion through the air causes a great resistance 

 in opposition to gravitation, which resistance has not yet, I 

 believe, been accounted for quantitatively on mechanical prin- 

 ciples. 



The explanation of soaring at great heights which presents 

 the fewest difficulties seems to me to be— that it is done by 

 means of upward currents. This has been suggested by several 

 observers, its main difficulty being the uncertainty that there are 

 such currents of sufficient strength. I shall try to show that 

 upward currents may be caused in two ways, but it would not 

 be possible to give a direct proof that the currents so arising are 

 strong enough. If, however, birds are seen to soar when one or 

 other of these causes is present, there is a strong probability that 

 they are true causes of soaring. 



Everyone who has watched the working of a windmill must 

 have seen that the force of the wind varies frequently, and some- 

 times rather suddenly. It is evident that there must be an 

 ascent of air in front of a current moving faster than the average 

 speed, and a descent of air behind it. As an example of this, 

 a cold south-west wind was blowing, with showers of rain at 

 intervals, accompanied, as often happens, by increased force of 

 the wind. I saw a flock of gulls soaring in front of one of these 

 squalls. There can, I think, be little doubt that there was an 

 ascending current, of which the gulls took advantage. 



Mr. W. Ferrel has shown (" Popular Treatise on the Winds ") 

 that if the rate of fall of temperature with increase of height 

 be greater than the rate of dynamical cooling of an ascendmg 

 current, the atmosphere is in an unstable state— that is, if by any 

 cause a mass of air be started in an upward direction in such an 

 atmosphere, the density of the ascending air is less than that of 

 the surrounding still air, so that the former would be driven 

 upwards, and an ascending current established, which would 

 tend to rush up to the top of the atmosphere if the instability, 

 consequent on the vertical decrease of temperature, should 

 extend all the way up ; but if the instability did not extend to 

 the top, then, at its limit, the impelling force would cease, and 

 friction would soon bring the ascending current to rest. Con- 

 versely, in an unstable atmosphere, if a mass of air be started 

 downward, the density of the descending air is greater than that 

 of the surrounding still air, and the descent tends to continue 

 down to the ground. Mr. Ferrel says (p. 440) : — " The unstable 

 state in unsaturated air occurs mostly on very dry and sandy 

 soils with little heat conductivity, when the weather is very 

 warm, and the heat rays of the sun are unobstructed by any 

 clouds above. The heat thus accumulates in the surface strata 

 of the soil and the lower strata of the atmosphere, and thus is 

 brought about the unstable state, at least up to a low altitude, 

 even in clear dry weather." And in speaking of what may be 

 called a multiple tornado (p. 412) : "As the tornado originates in 

 air in the unstable state, it often happens that there is about an 

 equal tendency in the air of the lower stratum to burst up 

 through those above at several places in the same vicinity at the 

 same time." 



This tendency of the lower strata to burst up in separate spots 

 may exist where the instability is much less than that required 

 to cause a tornado, as in the case of a plain strongly heated by the 

 sun, and in the absence of any gyratory motion round the centre 

 of an ascending current, there would bq no whirlwind, only a 



