453 



COMPARISON 



MEASURES OF LENGTH MOST GENERALLY USED FOR INDICATING ALTITUDES. 



It is too well known that the measures used in scientific researches among civilized 

 nations are not uniform, as the convenience of" all would require. In France the 

 metre is employed ; in England and North America, the yard and its third part, the 

 English foot ; in Germany, most commonly, the Old French or Paris foot, the sixth 

 part of the French toise called the Toise da Perou ; at the same time, however, 

 though not so extensively, the Rhine foot, in Denmark and Holland, and especially 

 in Prussia, where it has been declared, under the name of Prussian foot, the legal 

 measure in that kingdom ; in Austria, the klafter of Vienna and its sixth part, the 

 foot of Vienna; in Switzerland, the Swiss or federal foot, which has been adjusted 

 to the metrical system, and is three-tenths of a metre ; and so on. 



Tlie numerous altitudes ascertained, either by private efforts, or in connection 

 with the public works, and especially with the extensive geodetic opei'ations carried 

 on by the governments of these various countries for the survey of a regular map, 

 are expressed in the measures respectively adopted by each of them. These heights, 

 however, before they can be compared, require to be uniformly reduced to one 

 of these measures. Their relation to each other, therefore, is given here, together 

 with numerous reduction tables, designed to save both the useless expenditure of time 

 and the almost unavoidable errors arising from so numerous reductions. 



The exact relation of the standard measures above mentioned is not easily ascer- 

 tained, and the numbers given by the best authorities by no means always agree ; 

 for the manufacture of exact copies of a standard scale, and the accurate comparison 

 of it, require considerable skill, and belong to the most delicate operations of pliysics. 

 The numbers used for computing the following tables have been adopted, after a 

 careful review of the authorities, as the most reliable. A few words on the most 

 important original legal standards of measures may not be unwelcome. For further 

 details on the subject the reader is referred principally to Dove's work, Maas und 

 Messen, 2d edition, Berlin, 1835. 



The principal original, legal standards are the following: — 



1. The l^oise du Perou, tlie old French standard, made in 1735, in Paris, by Lan- 



glois, under the direction of Godin, is a bar of iron which has its standard length at 



the temperature of 13° Reaumur. It is known as the Toise du Perou, because it was 



used by the French Academicians Bouguer and La Condamine in their measurement 



E 7 



