124 LECTURE XII. 



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Avith otlier weights of known dimensions: but sometimes the flexure of a. 

 spring is employed for the comparison. Standard weights hav" generally been 

 deduced from a certain measure of a known substance, and in particular of 

 water. According to the most accurate experiments, when the barometer is 

 at 30 inches, and Fahrenheit's thermometer at 62°, 12 wine gallons of distilled 

 water weigh exactly 100 pounds avoirdupois, each containing 7000 grains 

 troy; and a cubic inch weighs 2524- grains. A hogshead of water, wine 

 measure, weighs, therefore, 525 pounds, and a tun 2100 pounds, which is 

 nearly equal to a ton weight. i\Ir. Barlow supposes that the tun measure of 

 water contained originally S2 cubic feet, and weighed 2000 pounds, which 

 was also called a ton weight, the gallon being somewhat smaller than it is at 

 present, and the cubic foot weighing exactly 1000 ounces, or 624- pounds. 

 A quarter of wheat weighed about a quarter of a ton, and a bushel as much 

 as a cubic foot of water. A chaldron of coals was also considered as equiva- 

 lent to a ton, although it now weighs nearly half as much more. But at the 

 mean temperature of this climate, or 52°, a cubic foot of distilled Mater 

 weighs only 9.9S ounces. The avoirdupois ounce appears to agree very nearly 

 with the ancient Roman ounce. Of the old French weight, 100 pounds 

 made 108 English pounds avoirdupois. The gramme of the'new weights is a 

 cubic centimetre of pure water at its greatest density, that is, about the 

 temperature of 39° of Fahrenheit; it is equal to 1 5^ English grains : hence 

 tlie chiliogramme is 2-j- pounds, and five myriogrammes are nearly a hundred 

 weight. Five grammes of silver, including one tenth of alloy, make a franc, 

 Avhich is one eightictli better than the old franc or livre, _^«tl is intrinsically 

 worth nearly ninepen^e three farthings English. 



The instruments usually employed for the comparison of weights are either 

 balances, or steelyards. In the common balance, the weights of the substances 

 compared are equal ; in a compound weighing machine, we use weights \vhich 

 are smaller, in a certain proportion, than those which they represent : in the 

 steelyard, a single weight acquires different values at different parts of tlie 

 arm, and in the bent lever balance, the position of the arms determines the 

 magnitude of the counterpoise. The spring steelyard measures the weight, 

 by the degree of flexure that it produces in a spring. 



The beam of a common balance must have its arms precisely equal. The 



