MECHANICS AND USEFUL ARTS. 17 



SIR JOHN IIERSCHEL ON THE FRENCH AND ENGLISH STANDARDS 



OF MEASUREMENT. 



The following is the substance of a letter communicated during the 

 past year to the London TIMCX by Sir Jolm Herschel, in reference to 

 the proposed introduction of the French metrical system into Great 

 Britain in place of the present English standard of measure. The 

 letter is especially noteworthy for the reason that its celebrated 

 author places himself in opposition to the general current of public 

 opinion on the subject. 



After stating that he considers the change on purely scientific 

 grounds to be a retrograde rather than an advance movement, he 

 proceeds to give the following as his reasons: " AVhatever be the 

 historical origin of our standard of weight, capacity, and length, as a 

 matter of fact our British system refers itself with quite as much arith- 

 metical simplicity, through the medium of the inch, to the length of 

 the earth's polar axis (a unit common to all nations), as the French 

 does through that of the metre to the elliptic quadrant of a meridian 

 passing through Paris (a unit peculiar to France). It does so 

 as regards our actual legal standards of weight and capacity with 

 much more precision than the French system, and as regards that of 

 length, (with a correction which, if legalized, would be absolutely 

 imperceptible, from the smallness of its amount, in any transaction of 

 life, and which can be applied, currcnte c alamo, almost without cal- 

 culation to any statement of lengths,) with even still greater, and 

 indeed with all but mathematical exactness. 



If the earth's polar axis be conceived divided into five hundred 

 million inches, and a foot to be taken to consist of twelve such inches, 

 then one hundred of our actual legal imperial half-pints by measure, 

 or one thousand of our actual imperial ounces by weight, of distilled 

 water at our actual standard temperature of 62 Fahr. will fill a hol- 

 low r cube having one such foot as its side. The amount of error in 

 either case is only one part in eight thousand. 



The theoretical French metre is one ten-millionth part of the ellip- 

 tic quadrant above mentioned ; the theoretical litre is one-thousandth 

 of a cubic metre ; and the theoretical gramme, one millionth part of 

 a cubic metre of distilled water at o2 Fahr. The actual error of 

 the French legal, or standard litre or gramme, or the deviation of 

 these standards as they actually exist, from their true theoretical 

 value, is one part in 2730, and is consequently relatively nearly three 

 times as great as the error in our standards of capacity and weight 

 when referred to the earth's polar axis as their theoretical origin in 

 the manner above stated. 



Our actual imperial measures of length deviate, it is true, by more 

 than this amount from their theoretical values so defined ; that is to say, 

 by one part in one thousand ; so that a correction of one exact thou- 

 sandth part subtracted from the stated amount of any length in imperial 

 measures suffices to reduce it to its equivalent in such units as corre- 

 spond to similar aliquots of the polr.r axis : a correction performed 

 if needed, as already remarked, instantcr and currente calamo re- 

 quiring no tables and almost no calculation. So corrected, the out- 

 standing error is onlv one part in 64,000. The actual legal metre in 



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