VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. 27 



the two results. It is further remarked, that the last week of July was so wet 

 as to occasion a total suspension of the operations on Hounslow-heath. On the 

 26th of that month, at 8^ a. m. the temperature being then 63°, the rods were 

 compared with the standard, and found to exceed it, at a medium, -^V part of 

 an inch. Now, if we suppose the whole base to have been measured with the 

 rods in that state, the difference would have amounted to more than 7-|- feet, 

 exclusive of what the standard itself might have altered from its original length. 

 Another comparison was made at Spring-grove, in the beginning of September, 

 by which it was found that the dew imbibed only in one night, or a space of 

 time not exceeding 14 hours, occasioned such an expansion in the deal rods, as 

 in the whole base would have amounted to 45.484 inches. It is sufficiently ob- 

 vious, that this last mentioned experiment was more accurate, in the proportion 

 of about 1 5 to 1 , than any comparison we could at that time have made with the 

 standard. But since immediately after it was finished, the sun shone out very 

 bright, it is by no means certain how soon the rods would again have contracted 

 to their former length, or near it, had they been exposed to his rays. Repeated 

 comparisons for ascertaining facts of this sort, at very short interims, are abso- 

 lutely incompatible with the nature of such tedious and troublesome operations 

 as the measurement of long bases; and here indeed lies the great objection to 

 the use of deal rods, that at no time can we be certain how soon, after a compa- 

 rison has been made, they may alter their length in a proportion, and sometimes 

 too even in a sense, different from what was expected. 



We then arrive at the description of the glass rods, or rather tubes, ultimately- 

 used to determine the length of the base. These were finally resorted to, as 

 less heavy and variable in their length, than metal rods, and not at all varying 

 by humour like the deal rods. Notwithstanding their great length, above 20 

 feet, and weighing about 6l lbs. each, they were found to be so straight that, 

 when laid on a table, the eye, placed at one end looking through them, could 

 see any small object in the axis of the bore at the other end. After a minute 

 description of the manner of preparing and fitting up these rods, we next arrive 

 at their application in the actual measurement of the base line. This was com- 

 menced on the 18th of August, both with the steel chain and the glass rods, 

 for a mutual comparison and check on each other. In this manner they pro- 

 ceeded, and in the course of the day were only able to measure the length of 10 

 chains, or 1000 feet. Being arrived at this point it was found, that the fine line 

 on the brass slide, marking the extremity of the 10th chain, fell short of another 

 fine line on the same slide, denoting the end of the 50th glass rod, just -^ of 

 an inch. Now it appears by the experiments with the pyrometer, what the real 

 contractions of the chain and glass rods were, for the degrees of difference of 



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