ON BALANCES. 49 



dards the kilogram, the unit of metric weight, and the 

 metre, the unit of metric length ; the object of the appoint- 

 ment of this commission having been to construct new 

 primary standard units of metric weight and length and 

 identical copies for all the countries of the civilized world. 

 You may see specimens of this platinum-iridium exhibited 

 by Mr. Matthey (Catalogue Appendix, Nos. 2596 and 259c), 

 showing also the peculiar sectional form of the new standard 

 metre. 



3. But I must proceed with a definition of weight. The 

 weight of a body is the measure of the force of gravitation 

 which the mass of our globe exercises upon the mass of all 

 smaller bodies upon its surface, and in a line perpendicular to 

 its surface, or, more properly speaking, to the surface of a 

 still liquid upon it. Gravitation is the effect of the force of 

 attraction which is inherent in all physical bodies, by which 

 they are drawn towards each other in proportion to their 

 mass, or the quantity of matter that each body contains. As 

 shown by Sir Jsaac Newton, attraction acts universally with 

 a force varying inversely as the square of the distance from 

 the centre of the mass, and with a velocity varying in propor- 

 tion as the medium through which the bodies drawn is more 

 or less rare. 



4. If our globe were a perfect sphere, and of uniform 

 density (I shall have presently to speak to you of density as 

 respects the density of bodies weighed), the force of gravita- 

 tion would be the same on all parts of the earth's surface. It 

 is known, however, that the figure of the earth is flattened 

 at the poles, and the best computations have determined this 

 flattening, or the difference between the length of its polar 

 axis as compared with its mean equatorial diameter, to be 

 about -o^th part of the earth's diameter. The effect of 

 this difference of distance from the surface to the centre of 

 the earth is to dimmish the force of gravity in passing from 

 the pole to the equator ; and it has accordingly been computed 

 that a weight of 100 Ibs. at the equator weighs more 100J 

 Ibs. at the pole, and more than 100J Ibs. in the latitude of 

 London. Practically, however, one could only test this dif- 

 ference of the force of gravity with a spring balance, and not 

 with a beam balance, where the weights in each pan must 

 both be equally affected by the force of gravitation, 



I shall presently show, when describing the mode of scien- 



VOL. II. E 



