210 SECTION MECHANICS. 



Another method of measuring matter is by its attraction towards 

 the earth, or, thirdly, the measurement of weight, represented by a 

 great variety of balances of ancient and modern construction. These 

 may be divided into beam weighing machines, which appears to be at 

 the same time the most ancient and the most accurate, into spring 

 balances and torsion balances. The accuracy obtained in 'weighing 

 is truly surprising, when we see that a mass of one ten-millionth part 

 of a gramme suffices to turn the scale of a well-constructed chemical 

 balance. Perfect weighing, however, could only be accomplished in a 

 vacuum, and, in accurate weighing, allowance has to be made for the 

 weight of air displaced by the object under consideration. The general 

 result is that the mass of light substances is really greater than their 

 nominal weight implies, and this difference between true and nominal 

 weight must vary sensibly with varying atmospheric density. 



Weighing in a denser medium than atmospheric air, namely, in 

 water, leads us, fourthly, to the measurement of specific gravity, which 

 was originated by Archimedes when he determined the composition 

 of King Hiero's crown by weighing it in water and in air. 



Among measures of weight may be noted a balance, which weighs 

 to the five-millionth part of the body weighed, sent by Beckers Sons, 

 of Rotterdam ; another from Brussels, weighing to within a fourteenth- 

 millionth part of the weight, in weighing small quantities ; a balance 

 formerly used by Dr. Priestly ; and Professor Hennessy's standards, 

 derived from the earth's polar axis, as Common to all terrestrial 

 meridians. 



Next comes, fifthly, the Measurement of Time, which, although of 

 ancient conception, has been reduced to mathematical precision only 

 in modern times. This has taken place through the discovery, by 

 Galileo, of the pendulum, and its application, by Huygens, to time- 

 pieces in the i7th century. The most interesting exhibits in this 

 branch of measurement are, from an historical point of view, the 

 Italian, German, and English clocks of the iyth century; the time- 

 keeper which was twice carried out by Captain Cook, first in 1776 

 and which, after passing through a number of hands, was brought 

 back to this country in 1843 ; and an ancient striking clock, supposed 

 to have been made in 1348 it has the verge escapement, which is 

 said to have been in use before the pendulum. The methods em- 



