rk 
Aug. 24, 1871] 
NATURE 
323 


insufficient optical power and bad definition, which disappear in 
a fair atmosphere with a good telescope well adjusted to focus.” 
With respect to facts, I must be allowed to observe that I be- 
lieve the facts are entirely the other way. This is a point which 
can only be tested by appealing to the facts themselves. 
Wales and Dymond observed the Transit of Venus in 1769 in 
the Hudson’s Bay. This is their account of what they ob- 
served :—‘‘ We took for the instant of first internal contact the 
time when the least visible thread of light appeared behind the 
subsequent limb of Venus, but before that time Venus’s limb 
seemed within that of the sun, and his limb appeared behind 
hers ‘n two very oblique points, seeming as if they would run 
together in a broad stream, like two drops of oil, but which, 
nevertheless, did not happen, but joined in a very fine thread at 
some distance from the exterior limb of Venus. This appear- 
ance was much more considerable at the egress thin at the in- 
gress, Owing, as we apprehend, to the bad state of the air at the 
time. We took for the instant of internal contact at the egress, 
the time when the thr.ad of light disappeared before the pre- 
ceding limb of the planet, from which time W. W. took notice 
that he had told about 245, when the limbs of the sun and Venus 
were apparently in contact ; a circumstance which he did not ven- 
ture to attend to at the ingress.” 
The observers evidently saw these phenomena both at ingress 
and egress. From the detailed account at ingress the definition 
must have beenvery good. Ihave print«d the whole passage, 
including the part which may be turned against my argument, 
that ‘‘the appearance was more considerable at egress, owing, as 
we apprehend, to the bad state of the air.’ That the appear- 
ance of sucha ligament seen under great atmospheric tremor 
may have been more striking, I can well believe, 
Again, Chappe writes :— 
“A lentree totale de Venus, j’observai trés destinctement 
le second phénoméne que avait été remarqué par la plus grande 
partie des astronomes en 1761, Lebord du disque de Vénus 
salongea comme s'il étoit attiré par le bord du Soleil. Je 
nobservai point pour I’ instant del’entrée totale celui ott le bord 
de Venus commengoit a s’alonger ; mais ne pouvant pas douter 
que ce point noir ne fit partie du corps opaque de Vénus, 
jobservai le moment ot il étoit a sa fin; de facon que l’éntrée 
totale ne peut éire arrivée plutot, mais peut-étre plus tard de 
deux ou trois secondes. Le point noir é:oit un peu moins obscur 
que Je reste de Venus. Je crois que ces! le méme phénomene 
que celui que j’observai a Tobolsk en 1761.” I might quote 
other extracts. The phenomena are noted at Wardhus at the 
egress. It is expressly stated by Cook and Green at Otaheite 
that the extinction of the thread of light between Venus and the 
sun was gradual, and that at Otaheite the observers did not note 
tie end at ‘the ingress and the commencement at the egress. 
Now, as a practical man, I would ask Mr. Newcomb are not 
these appearances observational facts? They appear to me so 
real, that, to admit their non-reality, would be the same thing as 
if we were to argue that if Wales, Dymond, and Chappe had put 
down in their observing books the times when the limbs of Venus 
first appeared in contact, instead of waiting until they could not 
see the slightest trace of any connection between the limbs of 
Venus ana the Sun, they would not have given earlier times than 
those which now appear in their journals ; or that if Cook and 
Green at Otaheite had given the times corresponding to the last 
appearance of any connecting ligament at the ingress, the times 
given would not have been later than those which now appear in 
their journals. It was from these considerations, which appear 
clear enough, that I have treated the Otaheite obseiyations as 
referring to a different phase from those of the Hudson’s Bay 
observers, and Chappe’s ingress observations. Simi ar remarks 
apply to the egress observations. You cannot talk of such ap- 
pearances being simple products of insufficient optical power 
and bad atmospheric circumstances. The appearances presented 
to and described by Wales and Dymond, even at the egress, 
took place according to their own estimation, which is the largest, 
within about asecond of arc. Such appearances could not be dis- 
criminated amongst with insufficient optical power, and under 
very bad circumstances of observing. Chappe particularly uses 
the phrase ‘‘trés-distinctement.” This point appears to have 
been overlooked by many who have written much upon the 
subject. With respect to experimental facts, I should, imdeed, 
esteem it a great favour, and I am sure that it would be important 
as bearing on our preparations for the Transit of 1874, if Mr. 
Newcomb can refer us to any experiments bearing upon this 
point. It will, however, be necessary to understand clearly the 
positions taken up. 

First, I assume the existence under sufficient illumination of 
irradiation, Secondly, I assume that the illumination of the sun 
is so great that under the ordinary circumstances of telescopic 
observation the optical enlargement of the sun’s disc due to this 
phenomenon is about 3”, the exact quantity will vary under 
different ciicamstances. The data upon which these assumptions 
are grounded are, amongst others, the experiments of Dr. 
Robinson, vol. v. Mem. Royal Astron. Society, and the eclipse 
discussions male from observations with the great equatorial of 
the Greenwich Observatory. Can Mr. Newcomb refer us to any 
experiments which have been made with a disc sufficiently 
illuminated to present under the cireum:tances of examination an 
Optical enlargement of 2", and in whi-h sufficiently powerful 
optical means have been employed to discriminate between the 
changes presented within 1" of arc, as asmall portion of the 
illuminated surface near the limb has been cut off by an opaque 
body ? 
I know of no such experiments. I do know that experiments 
were made at Paris by Wolf, in which ,the illumination of the 
disc was such that no sensible optical enlargement was exhibited. 
The results obtained had, therefore, no bearing on the question 
of irradiation; they were simply experiments on the dis- 
appearance of a small portion of a feebly illuminated disc. The 
results are such as any one conversant with the subject would 
have predicted with such a disc. The diameter of Mercury is so 
small that the appearance presented in a transit would not be so 
clearly marked as in a transit of Venus. Of the reality of the 
appearance of a connecting ligament in the transit of Mercury 
of 1868 I have no doubt, for I saw it. I would, with all due 
diffidence, give here a word of caution respecting discussions of 
these results. The phenomena under discussion, whether real 
or supposed, are presented only within a second of arc from the 
sun’s limb, It is perfectly useless, therefore, to appeal to upon 
this question any observations which have been made with in- 
sufficient optical means to subdivide a second of arc. 
The optical enlargement by irradiation is a function of the 
brightness, and can be made insensible by sufficiently diminishing 
that brightness. Unfortunately, however, when this diminution 
of brightness is carried to a very great extent errors in an exactly 
opposite direction to those of irradiation will come into play, 
similar, in fact, to the results of Wolf’s experiments. The ob- 
servations of Mercury on the sun’s disc in 1868 were made with 
very different optical means, and some very different methods 
were adopted fer diminishing the sun’s glare. If the observa- 
tions are put together without any discrimination upon these 
points some curious results will appear. Iam afraid that some 
gentlemen have been much misled by want of attention to these 
simple points. E. J. STONE 
Royal Observatory, Cape of Good Hope, July 19 
On the Age of the Earth as Determined from Tidal 
Retardation 
CONSIDERABLE discussions have taken place in the Geological 
Society and elsewhere in regard to Sir Wm. Thomson’s conclu- 
sion that had the earth solidified several hundred millions of 
years ago, when it must have been rotating at a much greater 
rate than at present, its form ought to be different from what it 
actually is. That is to say, there ought to be a much greater 
difference than there is between the equatorial and polar 
diameters. I observe that the discussion on this point has lately 
been renewed at the meetings of the British Association. 
Although I regard all the other arguments advanced by this 
eminent physicist in regard to the age of the globe, so far as I 
haye been able to follow his reasoning, as unassailable, yet I 
never could agree to this conclusion deduced from tidal retarda- 
tion. But, so far as I remember, I have nowhere seen stated 
what appears to me to be the real objection to the argument. 
The objection is as follows :— 
As the rate of rotation decreases under tidal retardation, cen- 
trifugal force must decrease also. The consequence, therefore, 
is that the sea must be slowly sinking at the equator and ris ng at 
the poles (see Pz. Mag. for May 1868, p. 382) | But denudation 
is also lowering the level of the land at the equator. Now the 
whole question concentrates itself into this, viz., will denudation 
lower the level of the land at the equator as rapidly 
as the sea sinks? This question, happily, can be answered. 
The method lately discovered of measuring the rate of sub- 
aerial denudation enables us to determine the rate at which 
the land at the equator is being lowered. We are enabled from 
