It was the 25
November 1820 when Everest reached Table Bay and the
chance to recover from the fevers to which he had succumbed.
Little is known of the year he spent there other than
the work he did on the meridian arc that had been observed
by M. l’Abbé de LaCaille some 70 years
earlier. Prior to leaving India he had discussed with
William Lambton, the apparent inconsistency between
the arc measured at the Cape by LaCaille when compared
with the results from various other measures around
the world. Both Everest and Lambton were familiar with
observing in the vicinity of large mountain masses and
they knew of the experiments of Pierre Bouguer who went
on the Peruvian Survey Expedition of 1735.[3]
Everest could not make any progress until he had obtained
a copy of LaCaille’s Journal from Europe and it
was July 1821 before it arrived. Unfortunately it gave
little numerical data to go on although he was able
to recover some of LaCaille’s station positions.
In addition to the base terminals there were four major
stations connected in two triangles. LaCaille’s
observations had suggested that a degree at 33º
S. of 57 037 toises was almost equal to a degree at
45º N. of 57074 toises or, in other words, the
earth was more flattened towards the South Pole than
towards the North Pole. Was that suggestion correct?
In visiting the sites he was struck by the nearness
of both the north and south ends of the Arc to mountain
masses. Everest considered that rather than any effect
at these stations cancelling out it was likely that
the reverse would have occurred. In which case the arc
would have been too great by the combined effects of
the mountains at each end of the arc. To quantify this
Everest intercompared various arc measures and from
knowledge of the earth’s semiaxes a and b he determined
values of the compression of the earth’s form.
In taking this to its final stage Everest quoted LaCaille’s
arc value from “a very old edition” of Hutton’s
Philosophical Dictionary [2], as most of his reference
works were still in India, and would appear to have
perpetuated a transcription error. The figure of 410
814 Paris feet quoted by Hutton should read 418 014
feet to agree with the measure quoted by LaCaille of
69 669.1 toise. The value is then erroneously turned
back to toise as 68 469-the value used by Everest. How
easy it is to perpetuate mistakes! [4]. Luckily the
effect of the error hardly changed Everest’s conclusion.
Where the figures he used gave a difference of 8.99”
to attribute to attraction, it should have been 9.15”.
From this Everest calculated a need to increase the
amplitude of LaCaille’s arc by 8.99” to
compensate for the possible effect of local attraction.
If this were done the arc would fall into harmony with
an ellipticity of 1/300.
By 31 December 1821 he was back in India ready to renew
his efforts on behalf of the Great Trigonometrical Survey
. He summed up his stay in South Africa as “...the
fine climate of which had most thoroughly renovated
my health...” [1] |
.gif)
