the Trigonometrical Operation. 139 
on the whole group, whereby it is kept perfectly fa ft and fe- 
cure from motion in carrying from one fituation to another. 
Art. V. Blocks of box wood and conical rollers under the 
feet fcrews . 
By referring alfo to Plate V. it will appear, that direftly 
below each foot there is fixed to the lower furface of the ma- 
hogany a fmall block of box wood, curvilihear in the direction 
of its motion. On thefe three blocks refts the whole weight 
of the inftrument, which neverthelefs can be moved circularly 
on them alone. But to render the motion perfectly eafy, three 
conical brafs rollers, placed fomewhat nearer to the center, are, 
by means of their refpeftive fprings and regulating fcrews, 
brought to a ft and receive fuch a proportion of the weight as 
it may be neceffary to lay upon them. The head of one of 
thefe fcrews, which give more or lefs aftion to the rollers, may 
be feen at D in the principal view of the inftrument Plate III. 
as well as in the plan and feftion Plate V* 
Art. VI. Screws giving motion to the whole infrument. 
By examining attentively the general view of the inftru- 
ment may be feen, in two politicos, the great fcrew with the 
flat ivory head, whereby the entire machine received a circular 
motion. In one, it is attached to the curbs, as when in ufe in 
1787; in the other, it is laid upon the mahogany bottom, as 
was the cafe the fame year every time it was carrried to a new 
fituation. But this ivory-headed fcrew having been found to 
aft by jerks in moving fo great a weight, and confequeiitiy to 
be troublefome in adjufting the inftrument to the fixed point, 
or that of commencement in meafuring angles ; it was there- 
fore laid afide in 1788, and another apparatus or clamp was 
T 2 adapted 
