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made with an invariable pendulum. 
From having carefully studied your works before leaving 
England, I had conceived myself to be sufficiently qualified 
to undertake a course of experiments at once. In this, 
however, I was mistaken ; and the consequence has been, 
that of two extensive series which I made at Valparaiso, neither 
is I fear sufficiently accurate to deserve your notice. The 
experience, however, which I gained in the course of these 
operations, enabled me ever afterwards to proceed with con- 
fidence. And here I may take occasion to suggest the ad- 
vantage which, on future occasions, would arise from having 
the whole experiment performed in England, by the person 
who is afterwards to repeat it abroad, not under the hos- 
pitable roof of Mr. Browne, to whose valuable assistance 
every one who has attended to this subject, is so deeply 
obliged, but in the fields, and with no advantages save those 
which he could carry with him. He would thus in good time 
discover omissions in his apparatus, which are not to be sup- 
plied abroad, and be aided in surmounting difficulties before 
he had sailed, as I did, beyond the reach of appeal. 
The first series of experiments, No. I. was made, as you know, 
in London. The next, which is marked No. II, was made thirty 
two miles and a half north of the equator, at one of the Gala- 
pagos, a group of islands in the Pacific, lying upwards of two 
hundred leagues west from the Continent of South America. 
It was intended that a station should have been chosen im- 
mediately under the line, but the ship being swept to leeward in 
the course of the night by a strong current, this object could 
not be effected without losing more time than circumstances 
admitted of being spent in that quarter. 
The spot chosen for the experiments lies near the extremity 
