^oo Mjior+Gencral Roy’s Account cf the 
By referring to the Platt, it will be observed, that two nar- 
row pieces of ivory, each faflened with two lmall fcrews, are 
inlaid into the upper furface of the rods, within one inch and 
a half of the extremities of the tippings. Theie ivory pieces 
received the fine black lines cut into them when the lengths of 
the rods were laid oft, in the manner hereafter to be mentioned, 
and accurately determined the intermediate diftauce of 30 feet y 
or 240* inches, the meal ure to he uled in the application by 
coincidences: whereas, in that by contacts, the fpace com- 
prehended between the extremities of the projecting lips of 
the tippings, is 243 inches. 
Immediately behind each ivory piece, a cavity is formed un- 
derneath, in the middle of the Item.. This receives a brafs 
wheel, about eight-tenths of an inch in diameter, whofe axis 
turns in the fork of a brafs' fp ring, fve inches long, faftened 
by a ferew to the under furface juft before the crofs feet. 
Thefe fprings are only of fuch ftrength as to permit the wheels 
to be forced up into the cavities by the weight of the rod, 
which, in its adjufted Rate, always refts entirely on the fur- 
faces of the two frauds that fupport its extremities. But 
when the rod is to be raifed from the ffands, then the milled- 
headed fcrews, projecting, above the upper furface, and {landing 
over the middle of the fprings, being brought to a£f, the 
wheels are thereby prefled downwards, and receive the full 
weight of the rod, which is then eafily moved backwards or 
forwards to its true pofltion, either of contact or coincidence. 
The crofs-feet, placed about 5I inches from the ends of the 
rods, and i% inch from the infertion of the truflings, are each 
about nine inches long, 1 \ broad, and nearly an inch in depth, 
having their lower furfaces level with that of tfle ftem. By , 
2 means 
