312 
which will amply. repay the technologist who con- 
sults it. 
In conclusion we may add that this little book, though 
far from being “complete,” or exhaustive of any one 
subject it treats of, is yet compiled with great care and 
discrimination by the Editor, and will be found of much 
value by those unable to consult larger treatises or 
original papers. eres 
Il Potentiale Elettrico nell? Insegnamento Elementare 
della Elettrostatica. Per A. Serpieri, Prof. di Fisica 
nella Universita e nel Liceo Raffaelo di Urbino. 
(Milano, 1882.) 
THIS treatise is an elementary exposition of the theory 
of the Potential in its application to Electrostatical Phe- 
nomena. It is founded, as we learn from the preface, on 
the authors l-ctures at the Raphael Lyceum of Urbino; 
and is intended for the use of the Lyceums and Technical 
institutes of Italy. It is well known to all who interest 
themselves in such matters that a promising young school 
of physicists has recently been springing up in Italy, and 
that those who wish to be abreast of their time can no 
longer neglect the Italian scientific literature. If the 
treatise of Prof. Serpieri may be taken as a fair specimen 
of the scientific instruction given in the secondary schools 
of Italy, it is clear that this harvest of physicists is due 
in no small degree to careful sowing. 
The work deserves its title of Elementary, inasmuch as 
nothing is demanded of the student beyond a knowledge 
of elementary geometry and algebra, and a slight ac- 
quaintance with trigonometry. The author is mistaken, 
however, in supposing that an elementary treatment of 
eiectrical theory has not hitherto been attempted; for 
the English work of Cumming, published some six years 
ago, is almost identical in its aims with his own. 
Although Cumming’s treatise is an excellent one in many 
ways, we cannot help thinking that the Italian one is 
better fitted for the purposes of elementary instruction. 
Prof. Serpieri appears to us to have happily kept the 
middle way alike between poverty and redundance of 
matter, and between excess of mathematical and excess 
of merely experimental detail. 
In the first four chapters are developed the relation 
between potential and charge, and the theory of lines of 
force and equipotential surfaces. The fifth, sixth, and 
seventh chapters contain the theory of capacity, of elec- 
trostatic induction, and of the measure of potential. The 
eighth chapter contains a short sketch of the centimetre- 
gramme-second system of uniis, now universally adopted 
in accordance with the decision of the Electrical Congress 
at Paris ; farther details on this all-important matter are 
given in one of the appendices, and a considerable 
number of numerical examples is providéd to familiarise 
the student with the practical use of the system. The 
last seven chapters are devoted to the theory of con- 
densers. Not only is the theory explained in a simple 
and interesting way, but abundance of experimental 
results and numerical illustrations are given to enable the 
learner to judge how far the mathematical theory repre- 
sents the actual facts. The account of the experiments 
of Villari on the heat developed in the electric spark 
under various circumstances is interesting, and would 
probably be new to most English readers. 
The main fault we have to find with Prof. Serpieri’s work 
is that he has a tendency to cite second-hand authorities 
where it would have heen quite as easy, more instructive 
for his youthful readers, and ore just to give the original 
sources. Again, why of all the results concerning specific 
inductive capacity should he quote (p. 69) those of Gordon 
only, which have been precisely the most questioned, and 
why on the same page should the results of Boltzmann 
for the specific inductive capacity of gases not be coupled 
with the name of their author? 
NATURE 
[ Fed. 1, 1883 
LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his correspondents. Neither can hz 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 
In your last number I observe an interesting letter on the 
‘* Hovering of Birds,” by Mr. Hubert Airy. In that letter he 
refers to an opinion which I have expressed, that this ‘‘hover- 
ing” capnot be accounted for by the mere supporting agency of 
an upward current of air. The writer quotes this opinion as it 
was expressed in a letter to you (NATURE, vol. x. p. 262). But 
he does not seem to have read the fuller explanation which 1 
have given on this subject in Chapter III. of the ‘‘ Reign of 
Law.” To that chapter I must refer your correspondent for an 
explanation, which shows that hovering can be, and is perpetu- 
ally accomplished under the ordinary conditions of horizontal 
currents of air. It is very commonly performed (especially) by 
the whole tribe of Terns, or sea-swallows, over the surface of 
the sea, where there are no hills or mountains to deflect aérial 
currents from the usual horizontal course. 
Mr, Airy himself uses words which indicate that this agency 
of upward currents is quite superfluous. He says: ‘‘It is easy 
to see that a bird, with the exquisite muscular sense that every 
act of flight demands and denotes, might so adapt the balance 
of its body, and the slope of its wing-surface to the wind, as to 
remain motionless in relation to the earth.” He prefaces these 
words by these others : ‘‘ given such a slant upward current.” 
But no such ‘‘ gift” is needed. The bird has only to slope his 
wing-surfaces to the current, and precisely the same effect is 
produced as if the current had been otherwise ‘‘sloped” up- 
wards against a horizontal wing-surface. Mr, Airy’s own letter 
contains an*excellent explanation of this correspondence. 
Cannes, France, January 29 ARGYLL 
WITH respect to Mr. Hubert Airy’s interesting note (vol. xxvii, 
p. 294), I beg to say that I have very often seen the kestrel 
hovering over the perfectly level meadows of Middlesex with 
obvious ease, where no undulation of the ground could possibly 
affect the currents of air, Of the twelve instances Mr. Airy 
enumerates, I see only six refer to hawks (species undetermined), 
so this fact must be taken into consideration ; the conduct of 
rooks and crows under such circumstances seems to me to come 
under quite a different category from that of hawks, and in some 
instances gulls, thus ‘‘ prospecting” for their prey. Mr. Airy 
does not ignore this aspect of the question, but I think that by 
confusing objective with subjective ‘‘ hovering ” he complicates 
his theory. Henry T. WHARTON 
39, St. George’s Road, Kilburn, N.W., January 27 
Action of Light on India-rubber 
Ir may be in the recollection of some of your readers, that in 
1876 I pointed out that the deterioration of ebonite surfaces was 
due to the combined action of light and air. Some time after- 
wards it was remarked to me that our laboratory (an old green- 
house) was too light, and as a result all our india-rubber tubes 
would rapidly deteriorate. This led me to submit some pieces 
of ordinary black india-rubber to the same treatment as the 
ebonite in the former experiments. On October 11, 1879, four 
pieces.of caoutchouc connector of 5 mm. internal diameter were 
taken, two were placed in test-tubes plugged with cotton-wool, 
and the remaining two inclosed in hermetically sealed tubes. 
One of the sealed tubes, and one of those plugged with cotton 
wool were placed in a dark drawer, and the other pair in the 
laboratory window, with a north aspect, and in such a position 
that they were not under the influence of direct sunlight in the 
summer. To-day the specimens were examined. Both the 
sealed tubes were found to be slightly moist inside, and on 
opening them an organic odour, like that of an india-rubber 
shop, was perceived. The caoutchouc which had been exposed 
to air and light, was covered with a thin brown coating, and on 
being bent this coating cracked ; the end which had been most 
| exposed to the light was rather brittle, and could not be stretched 
i 
