Measurement of a Bafe on Hounflow-Heath. 443 
Topper. In the lower parts, as well as in the crols braces, there 
are femi- circular cavities lined with broad-cloth, fitted to re- 
ceive the diameter of the tube, which refls in them, and is 
confiequently fupported at five different points. The upper 
end-pieces, having likewife femi- circular cavities fitted to em- 
brace the upper part of the tube, flip down upon it, when it 
has been, by repeated trials, .brought to its true pofition ; that 
is to fay, the axis of the bore into the fame flraight line, the 
cafe being all the while fupported by its extremities on two 
ftands only, in the manner in which the rods are applied in 
actual meafu remen t. The braces within the cafe have alfo 
their upper pieces, which, in like manner, apply clofely to 
the tube, and are fixed to the lower ones by means of fcrews. 
The whole together ferve only as flays to keep the tube in its 
true place from fhaking ; but without binding it however too 
clofely. Laflly, the mahogany collar glued to the pack-thread 
on the middle of the tube, being flrongly fixed by four fcrews 
to the middle brace, as may be feen in the fedlion, is that by 
which the tube is kept perfectly immoveable with refpedl to 
the middle of the cafe; while it is unconfined longitudinally in 
the cavities lined with broad-cloth every where elfe. 
Both ends of the tube are ground perfedlly fmooth, and 
truly at right-angles to the axis of the bore. That end, which 
in meafuring ufually lies towards the left-hand (fince mofl 
people will work the fcrew with the right) projects about 
jeven-tenths of an inch without the cafe, and is called the 
fixed end, becaufe the apparatus belonging to it is fixed. The 
other end towards the right-hand projects about nine-tenths 
of an inch, and, having a moveable apparatus, is called the 
moveable end. 
The 
