Measurement of a Bafe on Hounflow-Heath. 47 7 
. Feet, 
The apparent length is to be augmented by the 
excefs of the expanfion above the contradion of 
the glafs rods,, contained in the thirteenth column, 
of the general,: table - 4.1 867 inches, reduced to 
the heat of 6.2°, as has been ufually done.in former 
operations of this nature , . . # Q.3489* 
The apparent length is farther to be augmented 
by the equation for 6° difference of temperature of 
the ftandard brafs fcale between 62° and 68% this 
laft being the heat in which the lengths of the 
glafs rods were laid off =. 20.3352 inches, as de- 
duced from the experiments with the pyrometer 1.. 6946 
Hence we have the corred length of the bafe in 
the temperature of 62° reduced to the level of the 
lowermoft extremity near Hampton Poor-houfe, 27404,7925 
This laft length requires yet a fmall redudion 
for the height of this lowermoft end above the 
iman level of the lea, luppoled to be fifty-four' 
feet, or nine fathoms, 
y v o- . 
0.0706 
Hence the true or ultimate length of the bafe,*" 
leuuced to the level of the fea, and making a por- 
tion of the mean circumference of the earth,, 
becomes 
27404.7219 
As feme fmall degree of uncertainty remains with regard to 
this, laft redudion, it may not be improper to fay yet a few 
words on the principles that have been adhered to in making 
the computation. It will be remembered, that the meafure- 
ment was made 3 | feet above the furface of the heath, that 
being the height of the ftands whereon the rods were placed 
