Me(jfurement of a Bafe on Hounflow-Heath. 477 



Feet,. 

 The apparent length is to he augmented hy the 



excefs of the expanfion ahove the contradion of 

 the glafs rods, contauied hi the thirteenth column 

 of the general tahle — 4.1867 inches, reduced to 

 the heat of 62°, as has heen.ufually done in former 

 operationsof this nature . .. .. o.^^g.i 



The apparent length is farther to he augmented. 

 hy the equation for 6° difference of temperature of. 

 the {landard brafs fcale between 62"^ and 68*, this 

 laft being the heat in which the. lengths of the 

 glafs rods were laid off — 20.3352 inches, as de- 

 duced from, the experiments with the pyrometer i . 6946 



Hence we have the correal length of the bafe in 

 the temperature of 62° reduced to the level of the 

 lowermofl extremity^ near Hampton Poor-houfe, 27404.7925 



This laft length requires yet a fmall reduction 

 for the height of this lowermoft end above the 

 mean level of the fea, fuppofed to be fifty-four 

 feet, or nine fathomSj . .. , •- 0.0706 



Hence the true or ultimate length of the bafe, . 

 reduced to the level of the fea, and making a por- 

 tion of the mean circumference of the earth, . 

 becomes •. • • • 2.7404.7219 .■ 



As fome fmall degree of uncertainty remains with regard to 

 this lafl redu^ion, it may not be improper to fay yet a few 

 words on the principles that have been adhered to in making 

 the computation. It will be remembered, that the meafure- 

 ment was made'. 3I feet above the furfiice of the heath, that 

 being the height of the ftands whereon the rods were placed ;. 



and 



