MEASURE OE A DEGREE ON THE COROMANDEL COAST. 515 



And a mean of six comparisons 

 gave the excess of the old 

 chain above the new one* • • • 1T08 ditto. 

 25. The temperature was 87°. 



And a mean of two compari- 

 sons, crave. 1 1*00 ditto. 



Mean 1086 ditto. 



Hence it appears, that, at the conclusion of the base, the 

 old chain was longer than the new one 1 1 divisions of the 

 micrometer very nearly, so that it had increased, from being 

 in use, 2 divisions, or T £ T of an inch. 



These experiments were made with great attention, and 

 when either chain was stretched out by the weight, it was 

 carefully brought into a line in the coffers. 



As I had reserved the new chain for a standard, and know- Rateof expan- 

 ing the temperature at which it had been measured off in 

 London, I considered it an object to determine its rate of 

 expansion and contraction compared with the thermometers 

 which had been in use in measuring the base, since these 

 were but common ones, and might probably differ from those 

 made usa of by General Roy and others, who had deter- 

 mined the expansion of metals by the pyrometer; and I was 

 farther induced to do this, from seeing the great variation 

 among them, when the degree of heat became above one 

 hundred, which it generally was in the coffers every day be- 

 fore I left off. To avoid those irregularities arising from the Time allowed 

 expansions being checked by the resistance from the pres- J? Jj ™ftQ, 

 sure on the coffers, I chose the times of sunrise, and from tion. 

 one to two o'clock, P. M. for making the observations. 

 Sunrise in India is generally the coolest time of the twenty- 

 four hours, and the chain had during the night, on account 

 of the uniform state of temperature, full time to free itself 

 from any resistance. At the hottest part of the day likewise 

 there is a considerable time when the thermometers are 

 nearly stationary, which will afford time for the resistance in 

 the coffers to be overcome ; and it is necessary to pay parti- 

 cular attention to this circumstance, for the chain will be 

 perceived to lengthen often for nearly half an hour after the 

 thermometers are at their highest. 



I had 



