ij 00 M jor- Gcmral R o v * s /iLCcunt of the 



By referring to the Piatt it will be obferved, that two nar- 

 row pieces of ivory, eacli faftencd with two faiall fcrews, are 

 inlaid into the iipper furtace of the rods, within one inch and 

 a half of the extremities of the tipplngs. Iliefe ivory pieces 

 received the fine black lines cut into them when the lengths of 

 the rods were laid oil', in the manner hereafter to be mentioned, 

 and accurately determined the intermediate dlftance of 20 feet, 

 or 240 inches, the mcalure to be ufed in the application by 

 coincidences : whereas, in that by contacts, the fpace com- 

 prehended between the extremities of the projeding lips of 

 the tlppings, is 243 inches. 



Immediately behind each ivory piece, a cavity is formed un- 

 derneath, in the middle of the flem. This receives a brafs 

 wheel, about eight-tenths of an inch in diameter, whofe axis 

 turns in the fork of a brafs fpriiig, five inches long, faftened 

 by a fcrew to the under fiirface juft before the crofs feet* 

 Thefe fprings are only of fuch fl:rength as to permit the wheels 

 to be forced up into the cavities by the wreight of the rod, 

 which, in its adjufted flate, always refts entirely on the fur- 

 faces of the two ftands that fupport its extremities. But 

 when the rod is to be raifed from the ftands, then, the miiled- 

 hcaded fcrews, proje cling above the upper furfiKe, and ftanding 

 over the middle of the fprings, being brought to a61:, the 

 wheels are thereby prelled downwards, and receive the full 

 weight of the rod, which is then eafily moved backwards or 

 forwards to its true poiition, either of contact or coincidence. 



The crofs-feet, placed about 5§ inches from the ends of the 

 rods, and il inch from the infcrtion of the truflings, are each, 

 about nine inches long, if broad, and nearly an inch in depths 

 having their lower furfaces level with that of the flcm. By 



2 means 



