220 Captain Hall’s details of experiments 
ter’s Mate of the Conway, in Experiment No. IV, the length 
of the seconds pendulum comes out 39.03881, and the mean 
ellipticity The circumstances in this case, however* 
were not so favourable as those of the first series, being to 
one another in the ratio of 47 to 397, or nearly as 1 to 8. 
This arose from the change which took place in the weather 
at that period, the sky being overcast, the temperature fluc- 
tuating, and the rate of the clock unsteady. 
RIO DE JANEIRO. 
Two extensive series of experiments were made at this place, 
first by myself, No. V. and then by Mr. Foster, No. VI. The 
total number of the factors in the first case being 210, and in 
the second 452. The results agree with surprising exactness 
for operations entirely unconnected. The length of the se- 
conds pendulum by my experiments being - - 39.04381 
By Mr. Foster ----- 30.04368 
The mean ellipticity by my experiments is - - _ 
By Mr. Foster ------ — - — 
J 3 02 -37 
The circumstances in both cases were favourable, especially 
in the steadiness of the temperature, and the uniformity of 
the clock’s rate ; but as they were decidedly most favourable 
in the case of Mr. Foster’s experiments, I have no hesitation 
in considering his as the most entitled to credit. 
Mr. Foster is the Gentleman to whose co-operation I owed 
so much when observing the comet at Valparaiso ; an account 
of which, in a letter to Dr. Wollaston, has appeared in a 
recent Volume of the Transactions. His present work speaks 
sufficiently for itself ; but I should be doing him scanty jus- 
tice by confining myself to such a reference, without also 
