VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 623 



tered from the wind and sun. The plank and bar were supported on 5 of the 

 tressels, or tripods, belonging to the R. s., and the upper surface of the bar was 

 brought into an horizontal plane by means of screws and a spirit level. The 

 brass points on the upper surface of the bar were brought into a right line, by 

 stretching a silver wire along the top, and pressing the bar laterally with wedges, 

 till all the points fell under the wire. Part of the chain was then placed on 

 rollers, which rested on narrow slips of wood fixed on the side of the plank, 

 about 5 inches below, and exactly parallel to the bar; and while it was fastened 

 to an adjusting-screw near one end of the plank, it was kept straight on the 

 rollers by a weight of .56 lb. 



From the extremities of the 20 feet on the edge of the bar, 3 fine wires with 

 plummets were suspended, which were immersed in vessels of water, the wires 

 hanging so as nearly to touch the chain. One end of the chain being then 

 brought under its wire, by means of the adjusting-screw, a fine point was made 

 on the chain coinciding with the other wire. This part of the chain was then 

 shifted, and another 20 feet measured in the same manner; and the operation 

 continued till the length of each chain was thus obtained at 5 successive mea- 

 surements. The result was, that in the temperature of 514-°, i" which the ope- 

 ration was performed, the chain a was found to exceed 100 feet by 0.1 14 inches, 

 and the chain e, by 0.058 inches. Now, according to the table of expansions 

 in vol. 75, Phil. Trans., the expansion due to 1° Fahrenheit on 100 feet of cast 

 iron is 0.0074 inches, and that of the chain being 0.0075, their difi^erence is 

 0.0001, and therefore for 2^° it will be 0.00025; consequently, as the points 

 were put on the bar in the temperature of 54°, and the chains measured in 5 14.° 

 or 2i° less, their lengths in the temperature of 54°, agreeing with the points on 

 the bar, will be a = 100 feet -f- 0.1 1425 inc. b = 100 feet + 0.05825 inc. 



The comparison of the chains with each other, as related before, with this 



determination of their lengths, furnish the data necessary for the reduction of 



the base on Hounslovv-heath. The wear of b, in measuring 38 chains, appeared 



1.75 

 to be 14 divisions of the micrometer-head = — ^ =: O.O0673 inches: and the 



wear of a was 9.7 divisions = ~- = 0.0373 inches. 



Inches. Inches. 



Then, from the excess of a above 100 feet, viz. 0.11425, and of b 0.05825 



subtract half the wear , O.OI865 0.00336 



leaving O.O956 0.05489 



Txr ^ iu 1 a c iu I • • *u "i A = 100 feet -f. 095 6 inc. and 

 We get the lengths of the chams m the J ,- 



^ fc.oi f ^u A- { B = 100 feet -I- .05489 nic. the 



temperature 01 54 before they were used m > , , , . , 



,, . . ( lengths used m the reduction of 



the measurement, viz. \ , ° 



J the base. 



