217 
made with an invariable pendulum. 
tenth of an inch in diameter, and readily distinguishable from 
the Observatory. The screen being, moreover, made in the 
form of a box to receive the lamp, it became impossible to 
misplace the light. The instrument was brought down to 
this mark, and the level carefully examined before and after 
every observation, except with some stars which followed 
too close upon one another. The sun was fortunately ob- 
served at noon every day ; and as its rays were never 
allowed to touch any part of the instrument, or to enter the 
Observatory, except at the moment of noon, and then only 
through a small hole, I had reason to hope that none of the 
adjustments were at this observation ever deranged. As 
the great alternations in temperature alluded to above, might 
naturally be expected to cause fluctuations in the going of 
the clock, it was satisfactory to have a series of regularly, and 
frequently recurring tests, brought to bear upon this essen- 
tial particular. As the same precautions were observed at 
every station, this account of them will apply to the whole 
series of experiments. 
But in order to your forming no higher than a correct 
estimate of this insulated experiment, it is right I should de- 
scribe to you the peculiar circumstances under which it was 
performed. It was above all to be regretted that we were so 
much limited in time, that we could not engage in a fresh 
series, either at the same island, or on some other lying 
nearer the equator, the service upon which the Conway was 
employed, rendering it necessary that our stay should not be 
longer at the Galapagos than the 16th of January. Now, as 
we anchored at Abingdon's Island on the 7th at noon, there 
were barely nine complete days in which every thing was to 
be done. We had to search for a landing place, which occu- 
MDCCCXXIII. F f 
