426 The Metrical System 



the result that they have adopted in practice a common 

 system of linear measurements, simply and decimally sub- 

 divided, the unit of which is directly derived from nature. 

 The common ground is the sea. The table of measures 

 which was and is used at sea runs 100 fathoms are one 

 cable length, 10 cables are one sea mile; whence one sea 

 mile contains 1000 fathoms. Every nation used the nautical 

 mile, and every nation had its fathom. The noon entry in 

 the log of a ship gave, after the latitude and longitude at 

 that hour, the distance run since the previous noon, and this 

 distance was expressed in nautical miles and decimals of the 

 same, as at the present day. The nautical mile is the length of 

 one minute of arc (i') of the meridian, in the latitude where the 

 ship is. The length of the minute of arc varies within certain 

 limits according to the latitude. These limits are known, and 

 they can be appreciated by means of the following table, which 

 gives the length of one minute of arc on the meridian in 

 British fathoms of six feet, on different parallels : 



Latitude o 20 40 6o c 90 Mean 



Length of i', fath. 1007-6 1008-8 101175 IOI 5'i 1017-5 1OI 3"3 



The French seaman used the brasse, which is the equiva- 

 lent of the British fathom. The statutory length of the 

 fathom in different countries differed, because it was defined 

 to be six feet of the particular country, and the length of 

 the foot differed in nearly every country: but the seamen 

 of all countries had the same natural standard for it, namely, 

 the stretch of the arms ; consequently, no fathom differed so 

 much from the one-thousandth part of either the equatorial or 

 the polar nautical mile that it would have caused any incon- 

 venience if it had been made exactly the one-thousandth part 

 of any distance within these limits. 



Here, then, was a unit of length, the adoption of which 

 could offer no ground for international jealousy, and it must 

 be matter of surprise that it was not pressed on the com- 

 mittee by Borda, who had had considerable experience at 

 sea, and whose name is connected with some of the most 

 refined instruments used in nautical astronomy. Of course, 

 he may have done so and been overruled. 



