44^ Mo.j or 'General Roy's Account of the 



ftrengthen the cafes, but more particularly to prevent tliemi 

 from being rent when long expofed to the fun's rays in the* 

 £eld, the fuies are covered with brown linen laid on ver\r 

 fiiioothly, and carefully glued with thin glue, ufed as a Wronger 

 Icind of pafte, to which it may yet be neceilkry to add a coat 

 of oil paint. 



Each of the glafs rods, completed in the mariner above- 

 mentioned, weighs about fixty-one pounds. Their lengths 

 were afcertained by means of new brafs points placed in th© 

 great plank, the fpaces of forty inches being laid off, with the 

 iitmofl care, from the brafs fcale, when the temperature of all 

 had remained for the greater part of two days (Aiigufl 15th; 

 and 1 6th) at or very near 68°. For- this purpofe two brafs 

 reiSlangular cocks, whofe alternate furfaces had been previoufly 

 ground together, were placed upon the plank, fo as to bifecl: 

 the extreme dots ; in which fituation they prefented to each 

 other furfaces that were truly parallel. The rods being then feve- 

 rally placed between the cocks * (or, as was found to be a better 



methodj 



•■^ The firft of thefe.cacks, .or that to v\4iich the fixed button was applied, had 

 a hole in it exaftly of the height of the center of the button, and large enough 

 to permit the point of the micrometer fcrew to pafs through it, the faid fcrew 

 being fixed on the farther fide, or beyond the cock. Thus, while the tempera- 

 iure continued accurately at 68°, the fixed button^ or any other plane furface, 

 being brought up to the hole in the cock, and the micrometer point fcrewed {0 

 far as juft to touch it, the coincidence continuing in. the interim perfed, the exav^ 

 diilance of twenty feet was obtained between the point of the fcrew and the 

 fecondcock ; at which time the divifion anfwering to the index on the head of tire 

 micrometer was carefully noted. This being done, the cock with the hole was 

 removed from the plank, and the rods were feverally adjufled by being placed 

 between the point of the fcrew and the fecond cock. This fubftitution of the 

 micrometer point, inftead of the firllcock, was found neceflaryj becaufe, during 

 , the operation of adjullment, the temperature would fometimcs change a degree, 



generally 



•J 



