606 
NATURE 
[April 14, 1870 
round the sun, one of which is the earth we inhabit. But the 
resistance which the earth offers to the motion of the ether is the 
cause which converts this motion into that which gives the sen- 
sation of light, heat, &c., just in the same way that other matter 
in motion is transformed by resistance into heat and light. A more 
simple illustration of what I mean would be given by supposing 
a current of sea-water to give out a phosphorescent light only 
when an obstacle to its motion is introduced, such as a stone or 
stick, or when the waves of the ocean give out light by dashing 
upon a rock or the sea-shore. 
From this line of argument it will be concluded that the only 
causes which exhaust the sun’s energy are the several planetary 
and other bodies, moving in space, upon which the waves of 
ether dash, thus transforming their energy into the sensational 
forces of light, heat, &c. ; but the area of these resisting bodies 
is exceedingly small in comparison with the rest of space, in which 
the ether is acting by its own energy, and without coming in con- 
tact with any resisting or exhausting obstacle. 
It remains, then, to account only for the amount of the sun’s 
energy which is absorbed or transformed by these planetary and 
other bodies. Although we may have thus reduced the solution of 
this mighty problem to a narrower space, yet it is just as difficult 
to account for the maintenance of the exhausted energy occa- 
sioned by a single grain of sand moving in space as by all the 
planetary and other bodies together. 
Seeing that constant exhaustion or transfer of the sun’s energy 
does take place, although in a much less degree than would be 
the case if it were not confined to the moving bodies in space 
alone, it remains then to account for its maintenance. The first 
question we should ask ourselves is this: Is there any evident or 
known force tending towards the sun as a centre? The immediate 
reply will be gravitation ; and although in the present state of 
scientific knowledge it may be difficult or impossible to define 
what gravitation is, yet there cannot be a doubt that it is a 
force acting on all matter, with a tendency to carry all 
material bodies direct to the sun. As such force dashes into 
or upon the sun, it becomes in its turn transformed into light, 
heat, &c. It is indeed not improbable that future discovery may 
teach us that gravitation may have its origin from and bear some 
certain proportion to the resistance presented by the several bodies 
in space, which are illuminated by the sun’s energy; thus estab- 
lishing the beautiful law of light and heat being transformed into 
the force of gravitation—gravitation again into light and heat ; 
thus sustaining and maintaining, for all time, the sublime fountain 
of motion and life, thought, and every sensation and action that 
organic matter is able to experience. W. L. 
Sir W. Thomson and Geological Time 
THE strongest statement about the retardation of the earth’s 
motion of rotation by tidal friction, supposing the earth had been 
for so long a time provided with an ocean, is to be found in the 
appendix to a sermon preached by Professor Pritchard, F.R.S., 
then president of the Royal Astronomical Society, before the 
British Association at Nottingham. He there, in combating 
Darwin, says, ‘‘One million of million years ago, if the solid 
earth could then have been provided with an ocean, the length 
of the day would probably have been less than the flash of the 
hundredth of a second of time !” 
J announced to the Literary and Scientific Society of Notting- 
ham that this was an error in calculation, and based on a fallacy 
in reasoning ; and Mr. Pritchard withdrew the result, while main- 
taining the method, in a letter read to the meeting after a lecture 
on the subject that I subsequently gave. But I am informed that 
it has since been republished in its old shape. 
There is a still more amazing statement put forward in this 
appendix by the champion of Anti-Darwinism. Mr. Pritchard 
says he is familiar with the optical structure of the human eye. 
He dwells on the wonderful mechanism, and hints at the wonder- 
ful chemistry of it ; and quotes the well-known passage from 
Darwin (Ed. 1. p. 188) in which, while he gives up all attempt at 
showing gradation in the structure of the eye of Vertebrata, recent 
and fossil, yet he shows that in the Articulata the series is more 
complete. He quotes this, I say, to show that Darwin undertakes 
to explain by natural selection the structure of the Awman eye, 
which is precisely what he declines todo, ‘‘ Let us attend,” he 
says, ‘‘to the process of natural selection by which this mar- 
vellous organ is said to have come into being.” ‘ ‘‘T can see,” 
says Mr. Darwin, ‘‘no very great difficulty. . . in believing that 
natural selection has converted the simple apparatus of an optic 
nerve into an optical instrument as perfect as is possessed by any 
member of the great Articulate class,” i.e. as perfect as the human 
eye.’ Is not this amazing? 
Rugby, March 22 J. M. WILseNn 
The Moon’s Diameter 
WILL you permit me to say a few words on the interesting 
question raised by Dr. Ingleby in your last? The sun, moon, 
and all the heavenly bodies appear set, as it were, in the blue 
sky when the weather is clear; and as they are rarely visible 
unless when surrounded by at least a small space of blue sky, 
it seems to me that they will be naturally judged to be at the 
same distance from us that the sky is. But what is this distance? 
What, in other words, is the mean distance from which the blue 
light diffused or reflected from the air or vapour comes to us? 
Prof. Tyndall, who has devoted much attention to the causes of 
this blue appearance, may perhaps be able to tell us. The 
problem, of course, is rather an indefinite one, but an approxi- 
mate solution might assist us in determining the question. 
As to the heavenly bodies appearing larger when nearer the 
horizon, I shall leave some one else to settle the angular magni- 
tudes in the case. Mr. Abbott, to whom Dr. Ingleby refers, 
proves that the fact is not confmed to the heayenly bodies, but 
that portions of the sky seen under the same angle appear at 
least three times as large when near the horizon as when near 
the zenith (‘Sight and Touch,” pp. 136-7). But then, does 
the blue light come to us from the same mean distance when we 
look towards the zenith and when we look towards the horizon ? 
or does it come from a much greater distance in the latter case, 
and thus apparently increase the magnitude of a portion of it 
whose size remains unchanged? In other words, is the sky seen 
as a hemisphere, or as a much smaller segment of a spherical sur- 
face (the observer being at the centre, not of a sphere, but of a 
small circle, the plane of which coincides with the horizon) ? 
Most persons who look at a clear sky will, I think, adopt the 
latter alternative. It will be interesting to know if scientific 
research bears out natural impression in the case. 
Other solutions of the difficulty might undoubtedly be pro- 
posed. Association of ideas, which is now the favourite device 
for helping a lame dog over a style, might be called to the 
rescue, and with some plausibility. Clouds and birds— 
everything, in fact, that passes above us—are nearest to us and 
look largest when most elevated. Elevation is thus associated 
with comparative nearness, and approach to the horizon with 
comparative distance. It is, however, simpler, if correct, to 
maintain that we see the sky as it really is, and that the apparent 
distances and magnitudes of the heavenly bodies are determined 
by the fact that they appear to be set in the sky, not placed at a 
great distance beyond it. W. H. STANLEY Monck 
Trin. Coll. Dublin, April 2 
Heat Units 
THE science of heat, which is capable of being made and is 
rapidly becoming one of the most exact of the experimental 
sciences, seems to labour unnecessarily under an excessive variety 
of units of measurement. At present there are used— 
Units of Mass. Thermometric Degrees. 
Grain, 
Pound, Centigrade, 
Gramme, Fahrenheit. 
Kilogramme. 
Whence, evidently, there result e/g/¢ different thermal units, to 
all of which the common name ‘‘ unit of heat” is applied, or, at 
least, names inadequately distinctive. In the face of this it 
would really seem that some such suggestion as I here proceed 
to make must eventually be adopted. 
Define, fi?st, as follows:—A J¢herm is the quantity of heat 
necessary to raise the temperature of I gramme of water from 
o° C to 1° C. Secondly, 1 kilotherm = 10 hectotherms = 1000 
therms = . ,... , thus having kilotherm, hectotherm, &c. 
suggestively corresponding to kilogramme, hectogramme, &c., 
in name as well as in nature. 
Therms and kilotherms, which would probably alone be re- 
quired zin practice, would thus take the place of ‘‘ thermal 
units, centigrade,’ ‘‘gramme-water-units,” ‘‘ kilogramme-units 
of heat,” and others more or less lengthy and inexact at present 
to be found in writings on Heat and Energy. 
College Hall, St, Andrew’s, April 4. Tuomas MUIR 
