379 
The Bacteriology of Diphtheria. Including Sec- 
tions on the History, Epidemiology, and Patho- 
logy of the Disease, the Mortality caused by it, 
the Toxins and Antitoxins, and the Serum Dis- 
ease. By Drs. F. Loeffler, A. Newsholme, 
F. B, Mallory, G. S. Graham-Smith, G. Dean, 
W. H. Park. and C. F. Bolduan. Edited by 
Prof. G. H. F. Nuttall and Dr. G. 5. Graham- 
Smith, Re-issue with Supplementary Biblio- 
graphy. Pp. xx+718. (Cambridge Univer- 
sity Press, 1913.) Price 15s. net. 
Tue first edition of this exhaustive work was re- 
viewed in the issue of Nature for April 29, 1909 
(vol. Ixxx., p. 243). The editors point out in the 
present edition that the conclusions arrived at in 
the papers which have been published since the 
first appearance of the volume have mainly con- 
firmed the opinions advocated in it; and conse- 
quently they decided only to add a supplementary 
bibliography of eight pages, recording the most 
important work published since 1908. In many 
instances the contents and conclusions of the 
papers included in the bibliography are indicated 
sufficiently in their titles; in other cases a brief 
summary of their contents has been added. 
LETTERS TO THE EDITOR. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Narurr. No notice is 
taken of anonymous communications. | 
Migration Routes. 
Tue experience gained from flights on aéroplanes 
and from the behaviour of airships may throw some 
light on why migratory birds follow certain routes. 
Pilots in aéroplanes can easily see rivers and ponds, 
and these form better guides than roads and railways; 
main roads, now usually tar-coated, are not con- 
spicuous, while the lighter coloured by-roads are more 
easily seen. There is evidence that migration routes 
are often along coast lines and river valleys; these 
are most conspicuous features in an uninhabited 
country, and birds when flying in the daytime below 
the clouds could have no difficulty in following them 
by sight. 
When flying at night, or above the clouds, birds 
would be able to follow the coast-line by the sound of 
the waves breaking on the shore. Dr. Gadow be- 
lieves, both from theoretical considerations and from 
his observations, that birds have very acute power 
of hearing faint sounds. Thrushes apparently are 
able to detect earthworms by the noise they make 
just before they come out of their holes in the earth. 
Owls have remarkably well-developed ears, both ex- 
ternal and internal, and the silence of their flight 
perhaps has been partly developed to enable them to 
detect slight sounds. Birds no doubt appreciate the 
songs of their mates, and parrots have the power of 
reproducing sounds with great exactness. Dr. Gadow 
adds, that judging from the structure of the ear, most 
anatomists think that the power of hearing in birds 
is much inferior to that of mammals. He does not, 
however, agree with this opinion. 
Observations on sound from an aéroplane are impos- 
sible because of the noise of the engine and propeller. 
But from a balloon sounds can be heard easily. 
NO. 2300, VOL. 92] 
NATURE 
[| NOVEMBER 27, 1913 
have been heard at 4500 ft.; a gun at 
8200 ft.; a dog barking at two miles; a band playing 
at 11,800 ft.; a railway traim at 4900 ft.’ Other 
observers have noticed the barking of dogs, the crow- 
ing of cocks, and the bleating of sheep when high up. 
Mr. Griffith Brewer heard on one occasion the sound — 
of the sea breaking on the shore. He was over the 
English coast with an offshore wind and a calm sea — 
underneath him, and the sound he heard came from — 
the breaking of the waves on the French coast at least — 
twenty-five miles away. He was amongst the clouds 
in falling snow, and could see nothing. As the wind — 
carried him along over the sea the sound of the waves — 
gradually increased, and this was the only assurance 
of his continued approach to the French coast. 
Even in calm weather the sound of the waves would 
be easily heard by birds when at a considerable height. — 
Those who have lived a short distance inland are 
familiar with the sound from the shore on calm 
nights. When there is much wind the waves break-. — 
ing are not heard because of the sounds produced by 
the wind in the trees or buildings near, The intensity 
of the sound from a single source, such as a dog 
barking, will vary inversely as the square of the 
distance, but it the sound comes from a line instead 
of a point its intensity will only vary inversely as the — 
distance. Mr. Mervyn O’Gorman has pointed out — 
that this is one of the reasons which accounts for — 
the great distance to which the sound from the sea 
breaking on the shore will carry. 
Osborne Reynolds has discussed the refraction of 
sound caused by wind and also by the variation of the 
temperature of the air at different heights above the — 
ground.2. The refraction caused by wind reduces the 
carrying power of the sound to a place on the earth's: 
surface to windward. Usually the temperature of © 
the air falls with increasing height, and this reduces — 
the carrying power of the sound in all directions to 
places on the earth’s surface. When the direction of — 
the sound makes a large angle to the surface of the 
earth the intensity of the sound will not be reduced. — 
On one occasion during his experiments the calls — 
from the occupants of a boat were heard on a yacht 
more than five miles distant. In this case the direc-— 
tion was horizontal, and no doubt the conditions were — 
exceptionally favourable for the transmission of sound, — 
but we should expect the conditions generally to t 
good for the transmission of sound in an upward — 
direction, where there are no solid objects to make 
sound shadows. 
It seems then that birds can have little 
in following coast-lines by day or night. wae 
Migrating birds, however, can only follow rivers 
by sound when these are so wide as to have waves 
breaking on their shores or so rapid that sufficient 
noise is made by the water tumbling over rocks. 
Mr. Griffith Brewer tells me that at sight ponds 
and rivers are indistinguishable from grass fields even 
in bright moonlight, except that the surface of the 
water acts as a mirror in which the brilliant reflec- 
tion of the moon or even of a star is seen. This can 
only be an efficient guide to migrating birds on moon- 
light nights with a clear sky and when they are 
flying in such a direction that the image of the moon 
in the water is within their field of vision. Mos 
birds have their eyes at the sides of their heads,* and 
this would give them the power of watching the 
reflection of the moon in a river or sea when it is 
1 Report on Eight Balloon Ascents in 1862 by James Glaisher, F.R.S. 
B.A. Report, 1862, p. 490. “ . 
% See ‘‘ Papers on Mechanical and Physical Subjects," by Osborne= 
Reynolds, F.R.S., pp. 9 and 157. % 
3 Certain carnivorous birds have their eves more in front; birds follow 
the same general rule as other animals ; the eyes of the hunter are in front 
which must help him to see his prey, and the eyes of the hunted are at the 
side of the head to enable him to watch his pursuer. De 
People shouting 
difficulty 
