438 Major^Gmeral Roy's Account of the 



centers of the pipes terminating the hafe, reduced to the level, 

 of the lowefl-, or tl^iat at Hampton Poor-houfe, in the tempe- 

 rature of 63% being that of the brafs fcale when the lengths 

 of the deal rods were laid off. All this, however, fuppofes 

 three things to be abfohitely certain : tirrt", that tlie expaniioix 

 of the rods has been accurately eftimated; fecondly, that no. 

 enor has arifen fron?i the butting of the rods againfl each- 

 other, in order to bring them into contafL ; and, thirdly, that 

 no miftake of any kind has been committed 'in the execution^ 

 When we come to give the true length of the bafe, as ulti* 

 mately afcertained by means of the glafs rods, it will appear^ 

 that one or more of thefe three have actually taken place;: 

 although it is moil probable, that only the two firfl: fources o£" 

 error have contributed their fhare of the total difference be- 

 tween the two refults. But the difcuffion of this point muHr 

 be deferred for the prefent ; and I Ihall now finifh the fubjett 

 of the expanfion of the deal rods, by raentioni-ng two other 

 comparifons of them, w'hich lerve to (hew ffcill more obvioufly,, 

 how improper they are for very accurate meafurement!. 



It has already been remarked, th-nt the lail.week of July 

 was fo wet as to occalion a total HUpenfion of the operations on- 

 Hounflow-Heath. On the 26th of that month, at 8 h. A.M. 

 the temperature being then 63''', the rods were compared with; 

 the ftandard, and found to exceed it, at a, medium, one-fifteentH 

 part of an inch. Now, if we fuppofe die whole bafe to have- 

 been meafured with the rods in that ffate, the difference would; 

 have amounted to more than 7I feet, exclufive of what thft- 

 ftandard itlelf might have altered from its original length. 



The other comparifon was made at Spring-Grove, in the 

 beginning of September, after our operations on the heath had 

 been iiuilhed, and the deal rods with their apparatus depofited 



under 



*7 

 J 





