336 
NATURE 
_ [Feb. 8, 1883 
Drs. Martin and Moule have lately brought out a 
pamphlet in which full directions are given ‘‘how to dis- 
sect a Chelonian,’ and we are pleased to see from the 
preface that they intend to follow it by a series of similar 
works. It is almost a pity that so exceedingly specialised 
a reptile as a Chelonian should be the first they treat, 
especially as they intend to include a lizard in the series, 
for it is doubtless far better to begin the study of reptiles 
with the latter than with the former. 
The species on which the work is based is Pseudemys 
vugosa, but, as Dr. Martin states, ‘‘ the end in view is not 
to provide a monograph on any one species, but to show 
a student ‘how to dissect a Chelonian,’” the fact that, 
when dissecting another species, the description in the 
book cannot altogether be relied upon, makes the student 
examine everything carefully for himself. 
Without working through the anatomy of a Chelonian 
with the help of the book it is impossible fully to appre- 
ciate its value, but from the arrangement, accuracy, and 
clearness of description, it will doubtless preve a great 
boon to the young herpetologist. 
The only fault we can find with it is one under which 
the ‘Elementary Biology”? equally suffers, and that is 
the want of illustrations. There is a frontispiece with 
four rather rough woodcuts of the skull (to which no 
reference letters are given in the text), but this is all. 
Of late several illustrated students’ biological books, 
intended as guides for practical work, have been brought 
out, but most of these are so inaccurate as to be practi- 
cally useless. It is therefore to be regretted that a book 
by so eminent a teacher as Dr. Martin should be so 
poorly provided with figures, and we hope that the rest of 
the series will be more fully illustrated, as their value 
would be thereby greatly increased. 
Ferns of Kentucky. By John Williamson. (Louisville, 
Kentucky: J. P. Morton and Co., 1878). 
ALTHOUGH this little volume has only just reached us, it 
cannot be said to be out of date; for the number of 
popular works on ferns—those published in England 
excluded—is so small, that an addition to their number is 
at any time welcome to the fern-lover who has become 
well acquainted with the common British species, and 
would gladly increase his knowledge of the tribe. There 
is, indeed, no parallel in French or German literature to 
the number of fern-books which have been issued in 
England ; but it would appear from the volume before us 
that in America fern-hunting is as popular as it is among 
ourselves; for Mr. Williamson asks in his preface “‘ Who 
would think now of going to the country to spend a few 
days, or even one day, without first inquiring whether 
ferns are to be found in the locality ?”’ 
Mr. Williamson has given descriptions of the species 
found in Kentucky, and the letterpress is accompanied 
by sixty plates, in which the characters of the genera and 
the habit of each species is represented. The descrip- 
tions, although short and couched in simple language, 
seem carefully done; and the absence of pretence about 
the work does not render it the less attractive. Although, 
of course, primarily intended for local use, the “ Ferns of 
Kentucky ”’ contains much to interest the British lover of 
ferns. 
LETTERS TO THE EDITOR 
[The Lditor does not hold himself responsible for opinions expressed 
by hts correspondents. Neither can he undertake to return, 
or to correspond with the writers of, rejected manuscripts. 
No notice ts taken of anonymous communications. 
{The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space ts so great 
that it ts impossible otherwise to ensure the appearance even 
of communications containing interesting and novel facts. | 
Hovering of Birds 
I AM much obliged to his Grace the Duke of Argyll and to 
Mr. H. T. Wharton for the notice they have taken of my letter 
on the hovering ol birds. It may be hoped that the question will 
not be allowed to rest until it has received its quietus at the 
hands of some mathematician who shall tell us with authority 
whether it is possible, according to the received laws of me- 
chanics, for a rigid body, by any disposition of its surfaces, to 
remain motionless (relative to the earth) for a minute in mid-air 
in a perfectly horizontal wind. 
It is disappointing to find that the 3rd chapter of the ‘‘ Reign 
of Law,” to which the Duke refers me, contains no satisfactory 
explanation of the phenomenon of motionless hovering. We 
read there (p. 161, 5thedition, 1868). ‘* When there is a strong 
breeze, no flapping is required at all, the force of the wind sup- 
plying the whole force necessary to counteract the force of 
gravity.” This is hardly a sufficient explanation. Let us 
imagine a bird at rest in ahorizontal wind. Neglecting friction, 
the only forces acting on the bird are (1) the vertical force of 
gravity, and (2) the resultant of the air-pressures on the different 
surfaces of the bird caused by the horizontal velocity of the wind. 
These pressures may be resolved parallel and perpendicular to 
the general plane of the wings, and the direction of their 
resultant will vary with the slope of the wing-plane ; but in every 
case it will lie to rearward of the vertical, and therefore in every 
case there will be a resolved horizontal force pushing the bird - 
backwards. Yet in spite of this force the bird is to remain at 
rest! Shade of Archimedes ! 
Consider, again, what must be the necessary corollary to the 
Duke’s proposition. Ifa bird can remainat rest ina horizontal 
wind, it necessarily follows that in still air a bird can float hori- 
zontally without losing velocity. We do indeed see rooks and 
other birds float long distances descending on outspread wings in 
still air, and it is marvellous how slow is the descent, so great is 
the resistance of air to a plane surface when at every successive 
instant the plane surface covers a fresh body of air that has not 
yet begun to yield. But no one ever saw a bird maintain a 
horizontal floating flight in still air, Either the descent is con- 
tinuous, or the bird loses velocity. 
It might be wished, in a matter of such importance, involving 
as it does nothing less than the establishment of a miracle, that 
the Duke of Argyll were more precise in his statement, for I feel 
curious to know by what resolution of forces he would demon- 
strate the existence of a forward horizontal force to balance the 
backward force which he cannot deny to be present. A diagram 
setting forth the Duke’s views would be exceedingly welcome. 
My own diagram, I fear, will not serve his purpose any better 
than my words, in spite of the attempt he makes to press them 
into his service. The Duke says: ‘‘ The bird has only toslope his 
wing-surfaces to the [horizontal] current, and precisely the same 
effect is produced as if the current had been otherwise sloped 
upwards against a horizontal wing-surface.” Perfectly true, 
provided his Grace can tilt the direction of gravity through the 
same angle. Otherwise not (at the same time there is nothing 
about a ‘‘horizontal wing surface”’ in my letter). 
Mr. Wharton will do good service if, in recording any future 
observations he will note precisely the local circumstances under 
which they are made, bearing in mind that such an obstacle as a 
stack, a barn, a high hedge, or a thick tree might be enough to 
give the wind an upward throw. I presume he understands 
that the phenomenon to be explained is that of a bird remaining 
at rest in mid-air, with wings motionless, not fluttering. 
Woodbridge, February 5 HUBERT AIRY 
THis is a very interesting problem, and one which has been 
very clearly treated by Mr. Hubert Airy, so far as he dis- 
cusses it. 
I think, however, that birds are able to “‘hover” in other 
conditions than that he mentions, namely, where the wind is 
diverted upwards by blowing straight against cliffs or rising 
ground, 
The wind in this country at least is generally of a cyclonic 
character. Now such wind blows in toward the centre of 
depression, as has been shown by the Rev. W. Clement Ley, at 
an average angle of 25 degrees (inward from the tangent to the 
isobar). If we assume the wind to blow at a distance of 600 
miles from such centre at a rate of 20 miles per hour—on an aver- 
age for one mile in depth—then we have a volume of air converging 
upon the centre, which must rise into the upper regions there. 
If we assume, for the sake of obtaining a definite idea of this 
amount, that the centre of depression covers an area of 70 miles 
in diameter, say about 4000 square miles, then the air would rise 
vertically at an average rate of 400 feet per minute over the 
