128 DISCOURSE ON THE STUDY 



we are yet acquainted with, so that our means of 

 subdividing space are much inferior in precision. 

 The beautiful principle of repetition, invented by 

 Borda, offers the nearest approach to it, but cannot 

 be said to be absolutely free from the source of error 

 in question. The method of " double weighing," 

 which we owe to the same distinguished observer, 

 affords an instance of the direct comparison of two 

 equal weights independent of almost every source of 

 <>rror which can affect the comparison of one object 

 with another. It has been remarked by Biot, that 

 previous'to the invention of this elegant method, in- 

 struments afforded no perfect means of ascertaining 

 the weight of a body. 



(123.) But it is not enough to possess a standard 

 of this abstract kind : a real material measure must 

 be constructed, and exact copies of it taken. This, 

 however, is not very difficult ; the great difficulty is 

 to preserve it unaltered from age to age ; for unless 

 we transmit to posterity the units of our measure- 

 ments, such as we have ourselves used them, we, in 

 fact, only half bequeath to them our observations. 

 This is a point too much lost sight of, and it were 

 much to be wished that some direct provision for 

 so important an object were made.* 



* Accurate and perfectly authentic copies of the yard and 

 pound, executed in platina, and hermetically sealed in glass, 

 should be deposited deep in the interior of the massive stone- work 

 of some great public building, whence they could only be rescued 

 with a degree of difficulty sufficient to preclude their being dis- 

 turbed unless on some very high and urgent occasion. The 

 fact should be publicly recorded, and its memory preserved by an 

 inscription. Indeed, how much valuable and useful information 



