Thomson on British units

William Thomson (later Lord Kelvin) wrote the following note in 1887. The context was an article on the sun and he is explaining why he is not going to use British units.

A square metre is about 10 3/4 (more nearly 10.764) square feet, or a square yard and a fifth (more nearly 1.196 square yards). The metre is a little less than 40 inches (39.37 inches = 3.281 feet = 1.094 yards). The kilometre being a thousand metres, is a short mile as it were (0.6214 of the British statute mile). Thus in round numbers 62 statute miles is equal to 100 kilometres, and 161 kilometres is equal to 100 statute miles. The awful and unnecessary toil and waste of brain power involved in the use of the British system of inches, feet, yards, perches, or rods, or poles, "chains," furlongs, British statute miles, nautical miles, square rod (30 1/4 square yards)! rood (1210 square yards)! acre (4 roods), may be my apology, but it is only a part of my reason, for not reckoning the sun's area in acres, his activity in horse-power per square inch or per square foot, and his radius, and the earth's distance from him in British statute miles, and for using exclusively the one-denominational system introduced by the French ninety years ago, and now in common use in every civilised country of the world, except England and the United States of North America. The British ton is 1.016 times the French ton, or weight of a cubic metre of cold water (1016 kilogrammes). The French ton, of 1000 kilogrammes, is 0.9842 of the British ton. Thus for many practical reckonings, such as those of the present paper, the difference between the British and the French ton may be neglected.



JOC/EFR April 2007

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