JanuaRY 4, 1917] 
NATURE 
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349 
celestial globe. It is intended primarily for the 
determination. of bearings by observations of 
‘stars, for use in night marching. Briefly, it is 
a planisphere in which the principal stars which 
_are not too far from the equator are represented 
in a cylindrical projection; the star-chart is ad- 
justable for different dates, and there is a movable 
celluloid protractor on which are marked the pro- 
jection of the horizon and the projections 
of vertical circles at intervals of 10°. Fol- 
lowing the simple instructions given, the 
magnetic bearing of a star, even if its 
name be unknown, can readily be ascertained. 
The operations are entirely mechanical, and 
anyone of ordinary intelligence should be 
able to determine directions with considerable 
accuracy. The form of projection adopted, how- 
ever, has the defect of failing to give bearings 
of stars towards the north, and it is not very 
clear why the results are expressed in magnetic 
instead of in true bearings. A protractor adapted 
for southern Scotland and northern England, is 
obtainable alternatively or separately. 
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 Temperature Coefficient of Gravity. 
In Phil, Trans., May 17, 1916, Dr. P. E. Shaw 
published an account of a research from which he 
deduced that the gravitational constant is increased 
by one part in eighty thousand when the temperature 
of the larger mass is raised one degree. According 
to Dr. Shaw’s interpretation of the experiments, it is 
the mean temperature of tthe system which affects the 
coefficient of attraction, so that in the case of extremely 
unequal masses it is the temperature of the larger 
mass only which counts. The evidence does not seem 
strong enough to support so revolutionary a conclusion 
in view of the almost insuperable theoretical objections. 
One or two of the more obvious difficulties may be 
formulated in a few words. 
Take as an example the earth and a mass of 1 kg. 
Divide up the earth (ideally) into “ terrestrial particles ” 
of, say, I mgrm. each. When the temperature of the 
kilogram mass alone is raised one degree its attrac- 
tion for each “terrestrial particle’’ should be increased 
proportionally by 1-2x10-°. But by the same reason- 
ing the attraction between the kilogram mass and the 
earth as a whole should remain sensibly unaltered. 
In like manner, if we keep the kilogram mass at con- 
stant temperature and alter the temperature of the 
earth, the attraction between the kilogram and each 
“terrestrial particle” will be sensibly unaltered, while 
the attraction between the kilogram and the earth as 
a whole will have changed. This seems so essentially 
paradoxical that it is difficult to conceive of any supple- 
mentary hypothesis elastic enough to reconcile the con- 
tradiction involved. f 
The only way of avoiding this inconsistency is to 
admit that it is the product of the two values of a 
temperature-function which counts—i.e. that the tem- 
perature of the smaller mass is just as important as 
the temperature of the larger mass. Once this is 
admitted the experiments of Poynting and Phillips 
NO. 2462,' VOL. 98] 
prove that no variation exists greater than 10-° per 
degree Centigrade. 
It may be argued that the temperature of the attract- 
ing body determines the attraction—i.e. that action is 
not necessarily equal to reaction. In addition to 
violating the principle of momentum, this involves the 
possibility of constructing a perpetuum mobile, An 
elongated body, kept hotter at one end than at the 
other by means of ideal thermal insulation, would 
experience a resultant force in the direction of its 
length, and could be made to do work indefinitely by 
harnessing it like a horse to a mill. Is anybody pre- 
pared to believe this on any but the most conclusive 
experimental evidence? 
Again, it has been suggested by Prof. Barton that 
the temperature of the intervening radiation may 
determine the attraction. But the temperature of 
radiation is independent of the intensity, so that in- 
definitely feeble radiation would produce a finite effect. 
If the intensity of the radiation is substituted as the 
determining factor, it implies that the attraction of 
two bodies is increased if a beam of light passes be- 
tween them. If energy is to be conserved, this would 
imply that two bodies moving relatively to one another 
could increase or diminish the energy of a beam of 
light passing between them, and such a result would 
certainly be rather startling. Still more extraordinary 
would it be to find that a variation of o-o1 of a stellar 
magnitude on the part of the sun would change the 
length of the year by several minutes; yet this is what 
would be implied. There is no record of an appreciable 
change in the earth’s orbit caused by sun-spots. 
When one comes to examine the evidence out of 
which all these paradoxes arise, it can scarcely be said 
to be sufficient. Thus, for instance, as ‘‘indirect ex- 
perimental evidence,” Dr. Shaw cites Cornu, who 
found 5-50 for the earth’s mean density from winter 
work, and 5-56 from summer work, a difference of 
It per cent. To reconcile the sign of this variation 
with his own temperature coefficient, Dr. Shaw sug- 
gests that the apparatus in a laboratory may have a 
higher temperature in winter than in summer. He can 
scarcely have noticed that the excess of temperature in 
winter would have to be some goo degrees. Again, 
from Prof. Boys’s work on the gravitation constant, 
Dr. Shaw deduces a temperature coefficient of 10-°, of 
which, according to his own results, 98-7 per cent. 
must be ascribed to error. Can we have much con- 
fidence in the remaining 1-3 per cent. ? 
While we must all admire the experimental skill’ 
which enabled Dr. Shaw to observe a change of 
0-2 mm, at either end in a range of 200 mm., using 
a telescope and scale (especially when we know the 
difficulties he had to contend with), we can scarcely 
be expected to make these radical changes in our 
theories on the strength of such a very small effect. 
Though his reasons for rejecting experiments 
which gave a negative value for the temperature 
coefficient were no doubt excellent, the fact that such 
readings occurred is a little disquieting. Again, the 
readings vary amongst themselves by as much as the 
whole effect, and one knows how misleading a mean 
value of, say, 176-2, 175-9, 175-75 may be when the 
whole residual effect is only o-4 mm. 
_ In conclusion we should like to express our admira- 
tion for Dr. Shaw’s experimental work. We feel that 
as the result of such an elaborate research a null result 
is quite as important as, if less sensational than, a 
positive one. To thave reduced the apparent tempera- 
ture coefficient of gravity from the ro-? deduced from 
Prof. Boys’s measurements to one-eightieth of that 
value is certainly no mean achievement. 
F. A. Linpemany. 
C, V. Burton. 
South Farnborough, Hants, December 4. 
