
falls to the ground. The very life and soul of that defence was, 
inalmost his own words, that the disturbing forces produced the 
motion due to their action before friction ad time to act ; or, in 
other words, that the disturbing forces gave a pullso sharply and 
quickly that they did their work before friction, which the Arch- 
deacon looks on as rather a sluggard, could rouse itself and 
counteract them, that they were, in short, able to steal a march 
on friction each time they gave a pullor a push. 
Now it is clear that this explanation has no meaning in it, un- 
less the action of the disturbances is intermittent. Archdeacon 
Pratt admits that he never supposed that this is the case, there- 
fore he must find some new line of argument, if he wishes to 
continue his chivalrous defence of his old friend and, I believe, 
tutor, who can no longer speak for himself. I may add that all 
who knew Mr. Hopkins personally, and even those who, like 
myself, only knew of him through common friends, will appreciate 
-and admire Archdeacon Pratt’s championship, even if they are 
unable to agree with him. 
Tn his last letter (NATURE, August 31) Archdeacon Pratt has 
given a new and independent method, which leads him to the 
conclusion that the earth is solid from surface to centre, or nearly 
so ; so that if we accept his reasoning we must admit that, what- 
ever may be said of Mr. Hopkins’s method, his results at last 
are right. He mainly rests his argument on the consideration 
that such a limp thing as an earth with a crust not more than 100 
miles thick could not stand the strains with which the disturbing 
actions of the sun and moon are for ever trying it. 
How will it be, though, if we can show cause for believing 
that the crust of the earth is afterall somewhat of a limp thing, 
and yet does stand these strains? All a@ priori reasoning must 
give way to fact, if that fact can be established ; and though we 
may be surprised that so thin a crust is able to hold out against 
the violent treatment it has to undergo, yet, if we can show good 
reason for believing that the crust is, after all, thin, we must 
cease to wonder, and try to explain the seeming anomaly. 
I shall content myself now with putting forward one of the 
several grounds on which the thinness of the earth’s crust can be, 
I will not say established, but rendered highly probable ; and if 
my arguments shall prove in the end to have any weight, I have 
no fear that the seeming contradiction between them and the 
reasoning of Archdeacon Pratt and Prof. Thomson in the other 
direction will sooner or later be explained away. 
Everyone is familiar with what is known in Geology as Up- 
heaval and Depression, that over and over again during the 
earth’s lifetime portions of the solid crust have been raised, and 
others lowered relatively to a fixed datum, such as the sea level. 
Very naturally the idea springs up that the displacement is pro- 
duced by a thrust acting vertically upwards, or by a removal of 
some vertical support below. Some cases of small local up- 
heaval may have been brought about in this way, but this is not 
the machinery by which nature has acted on the large scale. 
The fact of upheaval is brought home to us when we find 
strata originally formed beneath the sea now high and dry far 
above its level ; how the upheaval was brought about we learn 
by recollecting that these strata were originally horizontal, noting 
whether they are displaced from that position, and, if so, after 
what fashion the displacement has taken place. Isolated obser- 
vations show us strata in some places horizontal, in others inclined 
at different angles to the horizon ; but when we combine into one 
‘view a large number of such observations, the result is that we 
see that the strata have been folded into troughs and arches, that 
when we find horizontal beds we are on the summit of an arch or 
the bottom of a trough, where inclined beds appear we are on the 
slopes. Further, we invariably find that the crumpling up of the 
strata has been most violent in those parts of the earth’s surface 
which have been raised highest above their original position, that 
is, on mountain chains. We come, then, to the conclusion that 
the way in which upheaval has been produced has been by a 
folding of the strata into troughs and arches, That the crust of 
the earth, instead of being a rigid unyielding mass, has been from 
time to time bent into folds, and, so to speak, crumpled up and 
wrinkled ; that it is not unlike, what it has often been compared 
to, the shrivelled skin of an old dried apple. 
Again, the supposition of a thin crust and an internal molten 
nucleus, gives a very satisfactory explanation of the way in which 
the crumpling is produced. As the nucleus cools it contracts, 
and the crust has to accommodate itself to the diminished sup- 
port within ; it cannot shrink, and therefore it gets crumpled, 
just as in the case of the apple, the inside shrinks more than the 
skin, and the latter in consequence wrinkles up. 
Here, then, is an argument in favour of no very great thickness 
NATURE 


[ Sept. 14, 1871 

and a certain amount of limpness in the earth’s crust, and it is 
not the only one of its kind; on the other side, are Archdeacon 
Pratt’s and Sir W. Thomson’s weighty reasons in favour of its 
rigidity. Far be it from me to attempt ‘‘ tantas componere lites.” 
I only wish to show that there are two sides, and two very good 
sides, to the question. 
What Mr. Hopkins has done seems to me to amount to this: 
he has shown that with a solid earth the amount of precession 
would be almost exactly what it is; but he has not shown that 
this would not equally be the case with an earth having a thin 
and a z/scous melted interior ; that case he has not attempted to 
handle, the case he did examine being that of a thin crust and a 
perfectly fluid interior. If if can be conclusively proved that 
the thin crust and viscous melted interior are incompatible with 
known astronomical and mechanical phenomena, we must give 
them up, but till that has been done we are bound to remember 
that, whatever has been said against them, there is something in 
their favour also. A. H. GREEN 
Barnsley, Sept. 8 
Temperature of the Sun 
I HAVE just seen the interesting note of Mr. Ericsson in the 
number of NATURE for July 13 (p. 204), and I am very glad that, 
this question should be thoroughly ventilated. 
Mr. Ericsson and others have been startled at the high degree 
of temperature at which I have arrived, and the appellation of 
extravagant is not spared. I beg leave, however, to observe 
that this conclusion does not materially differ from that obtained 
by Mr. Waterston. I am surprised that my opponents, satisfied 
with rejecting the result as extravagant, do not examine if the 
method is correct or not. The only objection that could be made 
is, that, while from the experiments of Soret, the resultant figure 
is 5,334,840 C., I doubled it, on account of the absorption 
which the radiation suffers in the solar atmosphere, whose integral 
effect is a great deal larger than Mr. Ericsson supposes ; and it 
would not be waste of time to discuss the experiments which 
prove that the absorptive power of the solar atmosphere is very 
considerable. Mr. Ericsson passes over this too slightly, saying 
that this absorption would be only oor of the whole, whie I 
have found it considerably greater. 
Mr. Ericsson refers to the explanation which I subjoined about 
this high temperature, that it is to be regarded asa virtual tem- 
perature, as if these were words which I would not attempt to 
explain. The explanation fwas, however, given very clearly in 
my own book, perhaps too shortly, since it seems not to have been 
understood. The word virtual was also employed by Mr. 
Waterston to indicate the degree of temperature which would be 
produced in a thermometer by the accumulated radiation of 
different transparent strata. And indeed this is not an absurd 
statement and incapable of conveying information, as Mr. Ericsson 
seems to suppose. 
First of all we must admit that a gas exposed to a radiating 
source does not always attain the same temperature as a solid 
body. Itis obvious, for instance, that the temperature of the 
free air at the top of a mountain is a great deal lower than the 
temperature of the thermometer exposed to the sun. This is 
due to the small absorptive power of the gas. Therefore, at the 
boundary of the solar atmosphere the temperature of the trans- 
parent gas may be a good deal lower than that of a solid ther- 
mometer (if by hypothesis it could preserve there its solidity). 
It is besides not incorrect to say that the different successive 
strata may add their own radiation, so that by two, three, or 
more radiating strata we could obtain a higher temperature 
than by a single one. At least this has been understood, 
even by M. Respighi, who, however, is of the same opinion 
as Mr. Ericsson about the exaggeration of my result. The 
integral effect of all the strata that contribute to this eleva- 
tion would be the indication of the thermometer, which may be 
higher than the temperature of a single outside stratum subjected 
to external radiation. 
Mr. Ericsson says that it is ofno consequence whether the sun’s 
photosphere belongs to the class of active or sluggish incandescent 
radiators. I think, however, this point to be very important. 
Since we cannot experimentally determine the temperature of the 
sun except by using its radiating power, it is very interesting to 
take into account this element as very substantial. Very few, 
indeed, will allow that which Mr, Ericsson takes for granted, that 


