52 Dr. Tiarks on the Longitudes 



957,000 pounds. Upon examining the experiments of Mr. 

 Barlow and of Mr. Tredgold on ash timber, it will be found 

 that the modulus of elasticity of the specimens tried by them 

 was 1,645,187 and 1,525,500 pounds respectively; so that the 

 jlcxihility of Earls-Barton ash, when compared with the wood 

 tried by Mr. Barlow and Mr. Tredgold, is nearly as 5 to 3. 

 The cohesion of Earls-Barton ash, as deduced from its trans- 

 verse strength, I found to be little more than 10,000 pounds 

 per square inch : but upon trying the cohesion by the direct 

 longitudinal force to pull it asunder, I obtained 24,700 pounds. 



Mr. Barlow gives the cohesion of ash 17,337 pounds, and 

 Mr. Tredgold 14,130 pounds ; the mean of these being 15,733 

 pounds : from which it appears that Earls-Barton ash is su- 

 perior to those just referred to in the ratio of 11 to 7. 



The ultimate deflection before fracture took place, according 

 to the formula of Mi\ Barlow, 1 found to be about 1\ times 

 greater than the ultimate deflection of ash in Mr. Barlow's 

 tables. 



Now if we estimate the toughness of wood to be in the com- 

 pound proportion of its cohesive strength and its ultimate de- 

 flection, we shall have "—^ : ^^|^ : : 12,350 : 3465 ; or in 



small numbers, 7 : 2 nearly; which shows that where tough- 

 ness is an essential quality, the ash growing in the neighbour- 

 hood of Earls Barton excells other wood of the same species, 

 and tried by Mr. Barlow, in the scale of 3^ to 1. 



I am. Gentlemen, yours truly, 

 Leighton Buzzard, Dec. 12, 1828, B. Bevan. 



VII. On the Longitudes of the Trigonometrical Survey of Eng- 

 land. By Dr. J. L. Tiabks, F.R.S. ^c* 

 IVrR. IVORY has proved (Phil. Mag. and Annals, July, 

 ^^ page 10) that, according to our present knowledge of 

 the figure of the earth, the difference of longitude between 

 Beachy Head and Dunnose ought to be 18" more than the re- 

 sult reduced in the Trigonometrical Survey; and he has sub- 

 sequently (Phil. Mag. and Annals, October, page 244) en- 

 deavoured to account for a part of this difference, by proving 

 an error in the formula by which that difference of longitude 

 was calculated from the data furnished by the Survey. Tlie 

 formula is this (retaining Mr. Ivory's symbols): 





Tang \ w = ^ ;,^v \ cotang --— 



CoinniunicateJ by the Author. 



