DECEMBER 17, 1903] 
MATURE 
151 
Translated 
im Zustand des Gleichgewichts.’’ Gauss. 
(Pp. 73» 
by R. H. Weber. Edited by H. Weber. 
price 1.20 marks.) 
No. 138.—‘‘ Uber die Bewegung der Korper durch 
den Stoss. Uber die Centrifugalkraft.’? C. Huygens. 
Edited by F. Hausdorff. (Pp. 79, price 1.40 marks.) 
Michta27 ae 
No. 20.—‘‘ Abhandlung tuber das 
Huygens. Second Edition. Revised by A. von 
Oettingen. This is a translation of the famous essay 
in which the wave theory of light was developed and 
the peculiar refraction of Iceland spar was investi- 
gated. (Pp. 115, price 2.00 marks.) 
No. 134.—‘t Experimental-Untersuchungen — uber 
Electricitat.”’ Faraday. Edited by von Oettingen. 
Sixteenth and seventeenth series, in which the source 
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103, price 1.60 marks.) : 
No. 136.—Ditto. Eighteenth and nineteenth series, 
describing his investigations on the development of 
electricity by the friction of water and vapour in other 
bodies, and on the relations of magnetism and light. 
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No. 21.—‘‘ Uber die Wanderungen der Jonen 
wahrend der Elektrolyse.’? Hittorf. _ First Part. 
Edited by Ostwald. Second Edition. (Pp. 115, price 
1.60 marks.) : 
No. 137.—‘‘ Abhandlungen zur Thermodynamik 
chemischer Vorgénge’’ (1869-1881). | Horstmann. 
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No. 139.—‘ Thermodynamische Abhandlungen uber 
Moleculartheorie und chemische Gleichgewichte ”’ 
(1867-1872). C. M. Guldberg. Translated and edited 
by R. Abegg. (Pp. 85, price 1.50 marks.) 
The last three numbers represent treatises which 
are familiar by quotation to all students of physical 
chemistry, and ought to be welcomed in this new 
ferm, 
Principii di Stereodinamica. 
Professor at Pisa. Pp. 
Hoepli, 1903.) 
STARTING with the formulze connecting the coordinates 
ot a particle of a rigid body referred to axes fixed in 
space with its coordinates referred to axes fixed in the 
body, the equations of motion of a rigid body moving 
in three dimensions are deduced from D’Alembert’s 
theorem. The applications include the problems of 
motion under no forces, in which the equations are 
integrated by elliptic functions, the simple and com- 
pound pendulum, motion of a billiard ball, &c., and 
Lagrange’s equations are also treated in this part. 
The second part deals with Hamilton’s principle in its 
various forms, and the third with Jacobi’s theorem. The 
work differs in many respects from ‘tthe conventional 
English text-books, in which special attention is given 
to the properties of moments of inertia and numerical 
examples rather than to rigorous deductions of the 
fundamental equations. Those whose lot it is to 
\ecture on ‘‘ three dimensional rigid ’’ will find a study 
of this book very useful and suggestive. G. H. B. 
The Fields of France. Little Essays in Descriptive 
Sociology. By Madame Mary Duclaux (A. Mary 
F. Robinson). Pp. vii + 318. (London: Chapman 
and Hall, Ltd., 1903.) Price 5s: net. 
Tus little book reveals the writer’s love for rural 
France, and her anxiety “‘ to set down chiefly the 
things I have seen for myself, or which have come 
under my own knowledge ”’ (p. 13), is reflected in the 
reality of the descriptions and in their sustained 
interest. Though there is little of an exciting 
character in the pages, readers who value word pic- 
tures of the habits and customs of country folk will 
find pleasure in this book. ’ 
NO. 1781, VOL. 69] 
By Gian Antonio Maggi, 
264. (Milan: Ulrico 
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 NaTURE. 
No notice is taken of anonymous communications. | 
The Velocity of a Nervous Impulse. 
Sir W. Gowers’s dilemma (p. 105) is of the library rather 
than of the laboratory, and I should hardly care to appeal 
to differing book-data by different observers in evidence of 
an acceleration of nervous processes during the last fifteen 
years, either in the same or in different individuals. 
I happen to possess records taken on myself in 1882 and 
in 1903, as well as upon my son, aet. fifteen, in May, 1903. 
In all three cases the velocity comes out at about 50 metres 
per second, as I read the records, but can easily be taken 
as indicating 60 metres if the rise of each curve from the 
base line is spotted a little differently. It is, in fact, 
advisable to examine the original data very closely before 
quoting velocities deduced from them, since very small 
differences in measurement along the abscissa multiply out 
to large differences of velocity expressed in metres per 
second. 
Thus in the instances enclosed, taken from my son last 
May in an interregnum from Greek roots, the times, as I 
read them, are 0-0063 and 0.0053 sec., and the velocities 51-5 
and 61-75 metres per second. 
/ 
SOI SBE IDOI 
May 3, 1903.—W. W. Waller, aet. 15; nerve-transmission velocity ; excita- 
tion above clavicle and at bend of elbow ; distance = 0°325 metre. Time 
diff. I, 070063 sec.; 2, 0'0053 sec. Velocity 1, 53 metres per sec. 
2, 62 metres per sec. 
The more carefully the records are taken and read the 
less inaccurately do the velocities come out. I think that 
Dr. Alcock’s estimate of 66 metres per second is a somewhat 
closer approximation to the truth than my estimate of 50 
metres, and a fortiori than the still lower estimate of 33-9, 
metres, which is that originally made by Helmholtz and 
Baxt in 1867. A. D. WALLER. 
A Useful Empirical Formula. 
THE very neat construction given by Prof. Perry in NATURE, 
December 3, p. 102, leads at once to the equation 
Ay _tanB Ax, 
ga. tania x)” 
5 pqte dy ax 
and the assumed equation y—a=6x" gives “= =7° 
y-a z 
Hence, approximately, pay. 
tan a 
Why does Prof. Perry prefer to write 
= log (1+tan B) 
log (1+tan a)’ 
which is less simple for computation ? 
11 Leopold Road, Ealing. J. D. EVERETT. 
