USED FOR INDICATING ALTITUDES. 455 



A standard scale made and divided by Troughton, and in all particulars identical 

 with Shuckburgh's scale, was brought to France in 1801 by Pictet. The comparison 

 of it with the standard metre, made by Prony, Legendre, and Mechain, gave, after 

 due reduction of the two standards to their respective normal temperatures, 



1 metre at 32 Fahr. 39.371 English imperial inches at 62 Fahr. 



This determination was adopted for all reductions in Kelly's Universal Cambist, and 

 in the French translation of the work, published in Paris in 1823. 



A new comparison was made with great care by Captain H. Kater, in 1818. (See 

 Philos. Trans, for 1818, p. 103.) The standards used were a brass scale metre, by 

 Fortin, terminated with parallel planes (metre a bouts), and a bar of platina on which 

 the length of the metre was marked by two very fine lines (metre a traits). Both 

 were compared with Shuckburgh's scale, and a double series of experiments gave 

 as the mean result : 



Brass metre at 32 Fahr. = 39.37076 inches of Shuckburgh's scale at 62 Fahr. 

 Platina metre at 32 Fahr. = 39.37081 " " " 



Mean 39.37079 " " " " 



On this value of the metre are based the reduction tables by Matthieu, published 

 yearly in the Annuaire du Bureau des Longitudes ; and it has come into general 

 use, both in Europe and in this country. 



Captain Kater gives besides, in the same paper, p. 109, note, the value of the 

 metre compared with Bird's Parliamentary standard as being 



1 metre at 32 F.= 39.37062 imp. inches of Bird's Parliamentary standard at 62 F. 



This value has been adopted by Dove, as being the legal one, in his reduction tables 

 in his work Maas und Messen, p. 175, etc., and by many German authorities. 



According to Bailey's experiments, made in 1835, when engaged in constructing a 

 new standard for the Royal Astronomical Society (Memoirs R. Ast. Soc., vol. ix.), 

 the value of the metre is (Lee, Collection of Tables and Formula, p. 62) 



1 metre at 32 F. = 39.370092 imperial standard inches at 62 F. 



The original legal standards having been lost in the fire which destroyed, in 1834, 

 the Parliament Houses, an act of Parliament provided for the construction of new 

 ones. An extensive and most careful comparison of the standards of length of 

 England, Belgium, Prussia, Russia, India, Australia, was made at the Ordnance 

 Survey office at Southampton by Capt. A. R. Clarke, R.E., under the direction 

 of Sir Henry James, Director, the results of which were published in London in 

 1866. This comparison gives the relation of the imperial standard to the metre as 



1 metre at 32 F. = 39.370488 inches of the imperial standard at 62 F. 



The value adopted in computing the tables in this volume, before this last com- 

 parison was made, is that determined by Capt. Kater in 1818, viz. : 



1 metre at 32 F. = 39.37079 English inches of the imperial standard at 62 F. 



The difference between these two equivalents of the metre is so small that, for 

 practical purposes, the substitution of Clarke's value, implying such laborious com- 

 E 9 



