95 



LECTURE XII, 



ON STATICS. 



THE examination of the magnitude of the various forces employed in 

 practical mechanics, constitutes the doctrine of statics. The term statics, 

 in a strict sense, implies the determination of weights only; hut it may 

 without impropriety be extended to the estimation of forces of all kinds, 

 especially active forces, that can be compared with weights, in the same 

 manner as the term hydrostatics comprehends every thing that relates to 

 the equilibrium of fluids. The measurement of the passive strength of the 

 materials employed, the changes produced in them by the forces which they 

 resist, and the laws of the negative force of friction, are also subjects imme- 

 diately introductory to the particular constructions and uses of machinery, 

 and nearly connected with the department of statics. 



The art of weighing is peculiarly important, as it furnishes us with the 

 only practical mode of determining the quantity of matter in a given body. 

 We might indeed cause two bodies to meet each other with known veloci- 

 ties, and from the effects of their collision we might determine their com- 

 parative momenta, and the proportion of their masses ; but it is obvious 

 that this process would be exceedingly troublesome, and incapable of great 

 accuracy ; we therefore recur to the well known law of gravitation, that 

 the weight of every body is proportional to the quantity of matter that it 

 contains, and we judge of its mass from its weight. If all bodies were of 

 equal density, we might determine their masses from their external dimen- 

 sions ; but we seldom find even a single body which is of uniform density 

 throughout ; and even if we had such a body, it would in general be much 

 easier to weigh it correctly than to measure it. 



The weight of a body is commonly ascertained, by comparing it imme- 

 diately with other weights of known dimensions ; but sometimes the 

 flexure of a spring is employed for the comparison. Standard weights 

 have 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 252 grains. 

 A hogshead of water, wine measure, weighs, therefore, 525 pounds, and a 

 tun 2100 pounds, which is nearly equal to a ton weight. Mr. Barlow * 

 supposes that the tun measure of water contained originally 32 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 62 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 equivalent to a ton, although it 



* On the analogy between English weights and measures of capacity, Ph. Tr. 

 1740, p. 457. 



