102 Mr. Topping's Account of the Measurement 
nity arise of comparing it, in future, with any measure of more 
acknowledged credit. The thermometer stood at 87°, when 
the rods were adjusted by it. 
The stands , which I had prepared for levelling the rods, 
were also brought out ; and it was with much regret that I 
found 1 could not profit by them, as I had hoped to do, assisted 
as I was by none but black people, in whom, I perceived, it 
would have been impossible, without incurring great loss of 
time, to have impressed a necessary idea of their nature and 
management. I therefore resolved to dispense with the stands,, 
and to lay the rods, end to end, upon the ground. It was in 
a similar way that the base line for a series of triangles, con- 
tinued throughout France, was. measured. The French rods, 
which were nearly of the same length and construction with 
mine, were disposed, in the very same manner, upon the rug- 
ged pavement of a highway near Paris ; so that I have every 
reason to believe the opportunity here afforded me, of a pecu- 
liarly level and sandy beach, to be the best of the two. 
The mode of conducting the measurement was this : — Staves 
were first set up, in a direct line, between the flags ; from every 
two of these staves a rope was occasionally stretched, as tight 
as possible, on the ground, and the rods were laid by the side 
of the rope. The first rod being properly placed, the second 
was laid near its end y and then very carefully adjusted, so as 
to touch the ferrule of the other, by a man, who had no other 
employment to engage his attention ; and in the performance 
of this office he was closely watched by myself. The ferrules, 
which were of thick brass, had been rounded, not only to make 
the contact more visible, but because the length of each rod 
was determined, by their having the spherical figure, more 
