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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 Princinle of Relativity 
In reading through Mr. Cunningham’s article on 
‘“The Principle of Relativity,” I have been struck by 
several points which seem to require some elucidation. 
Whether these difficulties are inherent in the ** prin- 
ciple of relativity’? as it is generally understood, or 
whether it is merely owing to the fact that for some 
time I have been accustomed to look at the matter 
from a different point of view, which I believe solves 
these difficulties, and that, on this account, I am 
rather obtuse towards other views, I cannot say. 
Some of these points may be best indicated by Mr. 
Cunningham’s own words, as follows :— 
(es ‘Now the very first thing that appears, if we 
accept the hypothesis of relativity, is that it is impos- 
sible for us to determine uniquely whether two events 
are or are not simultaneous.” 
(2) ‘‘ We find that the conception of ‘ simultaneity ’ 
does not become definite until we have assigned a 
definite velocity to a certain point.”’ 
Query—What is a ‘definite velocity’’? Is it to be 
defined in terms of length and time in the usual way, 
and, if so, how are length and time to be measured ? 
For, as Mr. Cunningham remarks :— 
(3) *““The next thing we may notice is that the 
notion of the ‘length of a body’ becomes indefinite 
along with the term ‘ simultaneous.’ ”’ 
(4) “If now we start from the fundamental law that 
there is a definite physically-determined velocity, that 
Otalpe tag a 
Query—What does Mr. Cunningham mean by a 
“ definite Pep seye -determined velocity ’ 
statements (1), (2), and (3)? 
Things apparently indefinite :—(1) ‘* Simultaneity ’ 
(2) mode of measuring length; (3) mode of mez isuring 
time intervals; (4) meaning of velocity. 
Query—What are Mr. Cunningham’s fundamental 
concepts ? A: A: Ross. 
Cambridge, June 20. 
in view of 
Ir should be fairly clear that the articles referred to 
by Mr. Robb were written with the intention of show- 
ing the need for a revision of the common ideas about 
space and time, which discussions on the principle of 
relativity have shown to be deeply ingrained. Mr. 
Robb would be the first to admit that such a revision 
is a necessity. The passages quoted above—(1), (2), 
(3)— were written to emphasise it. 
To Mr. Robb’s first query it must be replied that 
in the conceptual scheme of relations which we have 
evolved out of the data of perception, velocity 
defined in terms of length and time in the usual way 
but unfortunately experiment has not enabled us to 
think out a unique way of ‘measuring ”’ space and 
time. 
To the second query it need only be said that it is 
universal to think of light as being propagated in 
time, that this propagation is determined by physical 
considerations, and that it is at any rate a possible 
hypothesis that in the conceptual representation of 
the phenomena this propagation takes place always at 
a definite rate. j 
To the third query the reply is that the fundamental 
“concepts’’ in the representation of physical pheno- 
mena are oe and time. 
2331, VOL. 6O3) 
is 
NALURE 
[ JULY. 2, 19—4 
But the articles did not profess to describe in detail 
a logical scheme of the universe of motion. Mr. 
Robb’s forthcoming work in which this is attempted 
is anticipated with much interest. Ewe. 
Distribution of Rainfall on Sunday, June 14. 
I aM endeavouring to trace out the distribution ot 
the rainfall on Sunday, June 14, in a similar manner 
to that in which I investigated a thunderstorm some 
three years ago. May I ask anyone who is interested, 
and has not already communicated with me, to send 
as full details as possible either to me or to the 
British Rainfall Organisation, 62 Camden Square, 
N.W., unless they would report in due course to the 
Meteorological Office or to the Royal Meteorological 
Society ? 
Especially should I like information on the following 
points :—When the rain began; when it fell or did not 
tall; whether there was hail; if so, when and for how 
long; whether there was wind, and from what direc- 
tions. It would add to the value of the facts if the 
precise point of observation were stated. 
Information is desired as to the weather outside the 
storm area, as well as to the vee Jegs where rain 
fell. 
Any observations, however slight, even if they refer 
only to one particular time, will be welcome and useful. 
J. FATRGRIEVE. 
London Day Training College (University of 
London), Southampton Row, London, 
- June 26. 
WC. 
The Photo-electric Effect of Carbon as Influenced by 
its Absorbed Gases. 
THE existing contradictory 
electric effect: of carbon 
results on the photo- 
can be explained by means 
of the quantity and quality of the gases absorbed 
by the carbon. The infituences of ammonia, hydrogen, 
air, and carbon dioxide were investigated. The most 
consistent results were obtained ‘from carbonised. 
bamboo and hydrogen. Saturation curves showed 
ammonia to be the most active,.and carbon dioxide 
the -least. Distribution of velocity curves were ob- 
tained for bamboo and hydrogen. The maximum 
initial velocity was found to be independent of the 
quantity of hydrogen -absorbed, while the maximum 
current was proportional to the quantity of gas 
absorbed. : O. STUHLMANN. 
R. PIERSOL. 
University of Pennsylvania, June 17. 
MAYA ART-.! 
XCEPTING the splendid labours of A. P. 
Maudslay, embodied in four volumes of 
beautiful illustrations, with a descriptive text, the 
study of old Maya civilisation is almost entirely 
German-American, and it became a_ science 
through Foerstemann, who, with marvellous in- 
tuition, was the first to read some of the glyphs. 
It is a study quite self-contained, fascinating, 
but leading apparently nowhere. When_ the 
Spaniards conquered Middle America, the Maya 
glory was already a thing of the past, whilst the 
Mexican civilisation was at its height. This also 
has vanished without in the slightest degree 
1 Memoirs of the Peabody Museum of American Archeology and Ethnology, 
Harvard University. Vol. vi. A Study of Maya Art: Its Subject Matter and 
Historical Development. By H. J. Spinden. Pp. xxiii+285+29 plates+ 
map. (Cambridge, Mass; Peabody Museum, 1913.) 
