338 
On the Measurement of the 
[Nor. 
aided by the personal experience of Captain Everest, acquired during his connection 
with the grand survey in central India under Colonel Lambton. 
But, as many who take in our journal may have but slight acquaintance with the 
purpose or with the detail of trigonometrical measurements, we propose to make the 
subject more familiar by prefacing it, with a brief sketch, first of the historical course 
of this branch of knowledge, and then of the practical operations necessary in 
measuring a meridional arc. 
As soon as the astronomers of the ancient world had arrived at the conclusion of 
the globular form of the earth, it would seem an easy step for them to have for- 
med a rough estimate of its magnitude ; nothing further being requisite for this 
purpose than to know the travelling distance between two places situated north 
and south of one another, together with the difference of latitude as marked by the 
shadow of the gnomon of a sundial at each place. A long interval however, elapsed 
before any such speculations ensued. Nothing of the sort is to be met with in the 
astronomical works of the Hindus, and although Aristotle (350 B.C ) vaguely 
mentions, that the mathematicians of his day estimated the circumference of the earth 
at 400,000 stadia, the first who appears to have aimed at the solution of the problem 
on satisfactory data was Eratosthenes. From the distance between Syene and Alex- 
andria, he determined the degree to be equal to 700 stadia, and the circumference 
of the globe, therefore, to be 252,000 stadia, but his estimate is now of little avail, ai 
there exists no means of discovering the value of the stadium or unit of his sca.e- 
The next operation on record, and perhaps the first actual measurement of an arc of 
the meridian, was that undertaken by the Arabian sages under Caliph AlmAmdn, on 
the plains of Sinjar, in the year a. d. 827. They divided into two parties, one pro- 
ceeding to the south, and the other to the north, measuring their way until they 
found by their astrolabes that they had compassed an amplitude of two degrees; 
the accuracy thus attained by experiment fellshortof that acquiredbefore by the 6 R 
cian philosophers, tor it gave only 56$ miles to the degree; unless, as has been 
pected, an error has crept into the Arabic manuscripts consulted by the European 
astronomers. 
Se\ en hundred years intervened before the subject was taken up again, in ^ 
beginning of the 16th century. Fernel, in 1526, measured the distance from 
to Amiens, by counting the revolutions of his carriage wheel ; the difference o. 
latitude was already known, and fortunately Amiens lay almost due north of 1 j 
so that although this line has been, from its favorable situation, remeasure ^ 
verified no less than three times, very little change, as it happens, has been n e 
sary in Fernel’s original determination. . nt0 
Snellius in 1617, had the merit of introducing the method of triangulatin'^ 
geodetic measurements, which we shall have to describe more fully by a>^ 
A few years afterwards, Norwood measured the arc comprised between 
and k ork ; but his work shewed all the rudeness of a first attempt, and w as 
to the extent of 400 toises on the degree. Meantime greater progress was 
in France : Picard in 1665, had introduced the important improvement 0 
pic sights and micrometers to instruments for measuring angles, and ia ^ 
sured the Amiens’ line with an error of only 15 toises. Richer too had ^ 
iu 1672, the diminution of gravity towards the equator, by the alterat ^ 
rate of his astronomical clock at Cayenne, which made it necessary to re 
ength of its pendulum 1-lOtli of an inch. be e arth 
^ 1 le ^gaming ot the 18th century, the magnitude and figu re 0 ^ gffto o 
crossed more and more the attention of mathematicians. Huygens aD ^ 
1 engaged in the consideration of the question, and the latter e ® 
