454 COMPARISON OF THE VARIOUS MEASURES OF LENGTH 



of an arc of the meridian in Peru. What follows will show that it may almost 

 be called the only common standard, to which all the others are referred for com- 

 parison. 



2. The Metre is a standard bar of platina, made by Lenoir in Paris, which has its 

 normal length at the temperature of zero Centigrade, or the freezing point. Its 

 length is intended to make it a natural standard, and to represent the ten-millionth 

 part of the terrestrial arc comprised between the equator and the pole, or of a quarter 

 of the meridian. The length of this arc given by the measurement, ordered for the 

 purpose by the Assemblee Nationale, of the arc of the meridian between Barcelona, 

 through France, to Dunkirk, combined with the measurements previously made in 

 Peru and in Lapland, gave for the distance of the equator from the pole 5,130,740 

 toises, with an ellipticity of 3 ? , and for the length of the metre 443.29596 lines of 

 the toise du Perou, assumed to be 443.296 lines, or 3 feet 11.296 lines. This last 

 quantity was declared in 1799 to be the length of the legal metre, and vrai et 

 definitif, and is the length of Lenoir's platina standard. Later, and more extensive 

 measurements in various parts of the globe, however, seem to indicate that this 

 quantity is somewhat too small. The results of these various measurements, care- 

 fully combined and computed by Bessel, would make the quarter of the meridian 

 10,000,856 metres, and the metre = 443.29979 Paris lines ; Schmidt's computation 

 would make it 443.29977 lines, and both numbers are confirmed by Airy's results. 

 The legal metre is thus, in fact, as Dove remarks, a legalized part of the toise du 

 Pe"rou, and this last remains the primitive standard. But it must be added that a 

 natural standard, in the absolute sense of the word, is a Utopian one, which ever- 

 changing Nature never will give us. The metre is, for all practical purposes, what 

 it was intended to be, a natural standard ; though it must be confessed that, in 

 practice, the question is not whether, and how far, a standard is a natural or a con- 

 ventional one, but how readily and accurately it can be obtained, or recovered when 

 lost. 



3. The English Standard Yard is a brass bar, made by Bird in 1760, which was 

 declared, by act of Parliament, 1st May, 1825, the legal measure of length when at 

 the temperature of 62 Fahrenheit, under the name of Imperial Standard. Another 

 standard, sometimes also called Parliamentary Standard, was made by Bird in 1758. 

 Sir George Shuckburgh found both to be nearly identical, at least within 0.0002 of 

 an inch. (Philos, Trans, for 1798, p. 170.) 



Another scale of brass, however, made by Trough ton for Sir George Shuckburgh, 

 described in the Philosophical Transactions for 1798, and known as Shuckburgh's 

 scale, obtained among scientific men, perhaps, a higher degree of authority, on 

 account of the great accuracy of its division, and of its apparatus, devised by 

 Troughton, for delicate comparisons. The scale was used by Captain Kater, in 

 ' 1818, in his researches for determining the length of the pendulum beating a second 

 at London, and also the length of the metre, expressed in English inches of the 

 imperial standard. {Philos. Trans, for 1818.) 



Numerous attempts to determine the relation between the English and the French 

 measures show no inconsiderable discrepancies in their results. Omitting the older 

 comparisons with the toise, we give here the value of the metre in English imperial 

 inches as resulting from the most reliable comparisons. 

 E 8 



