1853.] OF THE ROYAL INSTITUTION. 243 
WEEKLY EVENING MEETING, 
Friday, February 4. 
W. R. Grove, Esq. M.A. F.R.S. Vice-President, 
in the Chair. 
G. B. Airy, Esq. F.R.S. Astronomer Royal. 
On the results of recent calculations on the Eclipse of Thales and 
Eclipses connected with it. 
Tue Lecturer commenced by remarking that he should not have 
thought the calculations connected with any other eclipse a subject 
worthy of his audience. The eclipse commonly called that of 
Thales is however one of extraordinary interest. It refers to a 
point of time which connects in a remarkable way the history of 
Asia Minor and the Greek colonies settled there with the history 
of the great Eastern empires. Its precise date has been for a long 
time a subject of discussion among the ablest astronomical computers 
and chronologers. It shews in a remarkable degree the power of 
astronomy ; for it is no small thing that we are able to go back so 
many centuries and confidently to describe a phznomenon which 
then occurred, almost to its minutest features. But it shews also 
the weakness of astronomy. It requires the combination of theory 
and observation, with a full sense of the possible inaccuracies of 
both, and with an endeavour by the use of each to correct the 
failings of the other. It requires general criticism, history, tradition, 
and a careful examination of geographical and military circum- 
stances. But when all these aids are properly brought to bear 
upon it, a conclusion is obtained upon which there appears to be no 
room for further doubt. 
In the last century, the computations, or rather the assumptions, 
of distant eclipses, were extremely vague. The theory of the 
moon’s motion, as applicable to distant eclipses, was imperfect; and 
it would almost seem that computers, under a sense of this imper- 
fection, felt themselves free to interpret the calculations as loosely as 
they might find convenient. Eclipses were adopted by them, as 
corresponding to historical accounts, which did not represent the 
physical phenomena when visible: some were even taken which 
occurred before sunrise or after sunset at the places of observation. 
The great step made in theory, in reference to these inquiries, was 
the discovery made by Laplace near the end of the last century, of the 
secular change in the moon’s mean motion in longitude (accompanied 
by similar changes in the motion of the perigee and the node). In 
explanation of this, the Lecturer pointed out that the force which acts 
