rOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. ^Q 



deal rods from the humidity of the air, which, by comparison with the standard," 

 was apparently most considerable in the 1st and 2d sections of the base, has 

 now wholly vanished; that is to say, the total amount of it has been over-rated 

 by 20.964 inches. 



After this follows a description of the microscopic pyrometer, invented by Mr. 

 Kamsden, and employed for determining by experiment the expansion of the 

 chief instruments concerned in the measurement of the base, viz. the standard 

 scale, the steel chain, and the glass rods, with experiments on the same. After 

 which the ultimate determination of the length of the base^ by bringing to ac- 

 count the several particulars, is given as follows: 



feet. 

 The hypothenusal length of the base, as measured by 1 369 .925521 



glass rods of 20 feet each + 4.31 feet, being the distance between 



the last rod and the centre of the north-west pipe, was 27402.8204 



The reduction of the hypothenuses to the horizontals O.0714 



Hence the apparent length of the base, reduced to the level of 



the south-east extremity, becomes 27402. 749O 



The apparent length is to be augmented by the excess of the ex- 

 pansion above the contraction of the glass rods, = 4.1 867 inches, 

 reduced to the heat of 62°, as has been usually done in former ope- 

 rations of this nature 0.3489 



The apparent length is further to be augmented by the equation 

 for 6° difference of temperature of the standard brass scale * between 

 62" and 68", this last being the heat in which the lengths of the 

 glass rods were laid off = 20.3352 inches, as deduced from the expe- 

 riments with the pyrometer *1.6946 



Hence we have the correct length of the base in the temperature 

 of 62^ reduced to the level of the lowermost extremity near Hamp- 

 ton Poor-house 27404.7925 



This last length requires yet a small reduction for the height of 

 this lowermost end above the mean level of the sea, supposed to be 

 54 feet, or 9 fathoms O.0706 



• There occurs a small oversight in this article, as was remarked in the Philos. Trans, of 1795, be- 

 ing an account of another measurement of the same base by Col. Williams, Capt. Mudge, and Mr. 

 Dalby. By which it appears that the equation for 6° difference of temperature above-mentioned, 

 should consist of the difference between the numbers for brass and glass, and not of that for brass 

 alone j that is, it should be 6° x (3.38938 — 1.41058) = 11.8368 inches = .9864 feet, instead of 

 1.6946 above employed, which made the base come out too much by 0.7082. This being deducted 

 from 27404.7219 the ultimate number above found, leaves 27404.0843, for what should be Gen. 

 Roy's length of the base as measured with the glass rodsj being only about 2| inches less than it waa 

 made by the other measures. 



