Il8 THE ADDRESSES, LECTURES, ETC., OF 



meet your eye, and upon which Mr. Merrifield will address 

 you. 



Another method of measuring matter is by its attraction 

 towards the earth, or, thirdly, the measurement of weight, repre- 

 sented by a great variety of balances of ancient and modern 

 construction. These may be divided into beam weighing machines, 

 which appear 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 weigh- 

 ing, allowance has to be made for the weight of the 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. 



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 fourteen-millionth part of the weight, in weighing small 

 quantities ; a balance formerly used by Dr. Priestley ; and Professor 

 Hennessy's standards derived from the earth's polar axis, as 

 common to all terrestrial meridians. 



"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. 



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 17th 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 17th centuiy, the 

 Timekeeper 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 



