Mccifurement of a Bafe on Hounflow-Heath. 44^ 



upper. In the lower parts, as well as in the crofs braces, there 

 are femi-circular cavities lined with broad-cloth, fitted to re~ 

 ceive the diameter of the tube, which refts in them, and is 

 confequently fupported at five different points. The nppcr 

 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 ftraight line, the 

 cafe being all the while fupported by its extremities on two 

 {lands only, in the manner in which the rods are applied iii 

 adual meafurement. 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 fliaking ; but without binding it however too 

 clofely. Laftly, the mahogany collar glued to the pack-thread 

 on the middle of the tube, being ftrongly fixed by four fcrews 

 to the middle brace, as may be feen in the fedion, is that by 

 'which the tube is kept perfectly immoveable with refpe£l: to 

 the middle of the cafe ; while it is un confined Ibngitudinaily ia 

 the cavities lined with broad-cloth every where eife. 



Both ends of the tube are ground perfe6tly 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) projedls about; 

 leven-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 projeds about nine-tenths 

 of an inch, and, having a moveable apparatus, is called the 

 moveable end. 



Th& 



