138 MEASUREMENT OP AN AnC ON THE 



SECTION I. An account of the Base Line. 



Some time had been taken up in examining the 

 country best suited, for this measurement, and at 

 length a tract was found near Sf. Thomas's Mount, 

 extremely well adapted for the purpose, being an 

 entire flat, without any impediment for near eight 

 miles, commencing at the race ground, and extend- 

 ing southerly. This being determined on, and the 

 necessary preparations made, it was begun on the 

 10th of April, and completed on the 22nd of May y 

 1802. 



I had expected a small transit in?trument from 

 England, for the purpose of fixing objects in the 

 alignement, and for taking elevations and depressions 

 at the same time ; but that instrument not having 

 arrived, 1 thought it unnecessary to wait, particularly 

 as the ground was so free from ascents and descents ; 

 I therefore used the same apparatus as I had formerly 

 done, viz. the transit circular instrument and the 

 levelling telescope fixed on a tripod with an elevating 

 screw in the center. In all horizontal directions, 

 this telescope fully answers the purpose, and as 

 there has been no deviation from the level to exceed 

 26' 30" excepting in one single chain, and those 

 cases but very few, I feel entirely satisfied as to the 

 accuracy of the whole measurement. 



The chain which was made use of is the one I 

 formerly had, and I was fortunate enough to receive 

 another from England, made also by the late Mr. 

 Kamsden, and this having been measured off by 

 the standard in London, when the temperature was 

 60° t:>y Fahrenheit's thermometer, it afforded me 

 an advantage of correcting for the effects of expan- 

 sion, a circumstance in which I was by no meansj sa- 

 tisfied in the former measurement. In order, there-, 

 fore, to have a standard at all times to refer to, I have 

 reserved the new chain for that purpose, and used the 



