ox DRAWING, WRITING, AND MEASURING. IOl> 



" The length of the pendulum, and that of a meridian of the earth, are the 

 two principal standards thdt nature affords us, for fixing the unit of linear 

 measures. Both of these being independent of moral revolutions, they can- 

 not experience a sensible alteration without very great changes in the physical 

 constitution of the earth. The first method, which is of easy execution, li?,s 

 the inconvenience of making the measure of length depend on two elements, 

 heterogeneous with respect to itself and to each other, gravitation, and time ; 

 besides that the division of time into small portions is wholly arbitrary. ' It 

 was resolved, therefore, to employ the second method, which, " says Mr. La- 

 place, " appears to be of very high antiquity; it is so natural to man to refer 

 measures of distance to the dimensions of the globe which he inhabits, in 

 order that, in transporting himself from place to place, he may know, by the 

 denomination of the space passed through alone, the relation of this sj)ace to 

 the entire circumference of the earth. This method has also the advantage of 

 making nautical measures correspond at once with celestial ones. The navi- 

 gator has often occasion to compare with each other the distance that he has 

 passed over, and the arc of the heavens corresponding to that distance; it is 

 therefore of consequence that these measures should be readily obtained from 

 each other, by altering only the place of the units. But, for this purpose, 

 the fundamental unit of linear measures must be an aliquot, part of the ter- 

 restrial meridian, which must correspond to one of the divisions of the circum- 

 ference of a circle. Thus the choice of the metre was reduced to that of the 

 xmit of angular measure, and the right angle, as constituting the limit of the 

 inchnation of two lines to each other, was considered as entitled to the pre- 

 ference. 



" The arc, Mdiich was measured in 1740, from Dunkirk to the Pyrenees, 

 might have served for finding the magnitude of the quadrant of the meridian ; 

 but a new and more accurate measurement of a larger arc was more likely to 

 excite an interest in favour of the new measures. Delambre and M^chain 

 were therefore intrusted with the direction of the operations for measuring 

 an arc from Dunkirk to Barcelona, and after making a proper correction for 

 the ellipticity of the earth, according to the measurement of the arc in Peru, 

 the quadrant was determined to be equal to 5130740 of the iron tojse used at 

 the equator, its temperature being 6] ■^° of Fahrenheit: the ten millionth part 



