Mr Galbraitli on Trigonometrical Surveying, c$r. 327 



that adopted by the French commission of weights and mea- 

 sures, as the ten-millionth part of the quadrantal arc of the 

 meridian. The difference of these amounts to 0.025 of a line, 

 or about ¥ £ 5 of an English inch — aveVy considerable quantity 

 in nice operations. Hence the French metre, if lost, cannot 

 be found again by taking accurately the ten-millionth part of 

 the quadrant of the meridian. It must be taken from the 

 standard metre of the French Archives, just as our yard must 

 be taken from our standard yard, or from its copies, since the 

 original was lost when the late Houses of Parliament were 

 burned. 



It is certainly of great consequence to possess a uniform 

 standard of weights and measures, which should be adopted 

 with caution and enforced by legal penalties, otherwise these 

 changes tend to render confusion worse confounded. Previous 

 to the late act relating to weights and measures, we had one 

 generally admitted Scotch acre, containing in round numbers 

 6084 square yards, which, since the passing of that act, has 

 been accounted 0104 square yards, especially about Glasgow 

 and the west of Scotland. Hence the direct consequence re- 

 sulting from the passing of that act has been to augment the 

 variety of our measures instead of reducing them. 



Mr Airy, indeed, the Astronomer-royal, determined also 

 some time ago the values of the earth's semi-axes, in an article 

 on the magnitude and figure of the earth in tbe Encyclopaedia 

 Metropolitana, and these, though exceeding all others of the 

 greatest authority by about 1000 feet, have been adopted by 

 Sir John Herschel in his Treatise on Astronomy. Notwith- 

 standing my respect for the authority of both these names, I 

 believe they can hardly be put in competition with that of M. 

 Bessel on such a question, and as my results agree nearly with 

 his, I may be excused, in the present state of our knowledge, 

 for still adhering to them. 



In determining the heights of mountains trigonometrically, 

 these tables must be employed either singly or in combination 

 with the effects of terrestrial refraction. For most ordinary 

 purposes the mean effect of refraction, generally taken at 0.08, 

 or vn . of the intercepted arc, may be thought sufficient, when 

 simultaneous and reciprocal observations cannot be obtained 



