ANNUAL ADDRESS OF THE PRESIDENT. LV 
requires time to recover. Is it not probable that an effect similar 
in kind, although less in degree, occurs in all solids when their 
temparature is varying? When we look at the highly polished 
terminals of an end standard, we are apt to regard them as mathe- 
matical surfaces, separated by an interval which is perfectly definite, 
and which could be measured with infinite precision if we only had 
the necessary instrumental appliances; but is that a correct view? 
The atomic theory answers emphatically, No. According to it, all 
matter consists of atoms, or molecules, of a perfectly definite size, 
and with definite intervals between them ; but even if that is denied, 
the evidence is now overwhelming that matter is not homogeneous, 
but possesses a grain of some kind, regularly repeated at intervals 
which cannot be greater than one 2,000,000th nor less than one 400,- 
000,000th ofaninch. Accordingly, we must picture our standard bar 
as a conglomeration of grains of some kind or other, having magni- 
tudes of the order specified, and all in ceaseless motion, the ampli- 
tude of which depends upon the temperature of the bar. To our 
mental vision the polished terminals are therefore like the surface 
of a pot of boiling water, and we recognize that there must be a 
limit to the accuracy with which the interval between them can be 
measured. Asa basis for estimating how near this limit we have 
approached, it will suffice to say that for fifty years past it has been 
customary to state comparisons of standards of length to one 1,000,- 
000th of aninch. “Nevertheless, most authorities agree that although 
one 100,000th of an inch can be distinguished in the comparators, 
one 20,000th of an inch is about the limit of accuracy attainable 
in comparing standards. Possibly such a limit may be reached 
under the most favorable circumstances, but in the case of the yard 
and the meter, which are standard at different temperatures, the 
following values of the meter by observers of the highest repute 
render it doubtful if any thing like that accuracy has yet been at- 
tained. 
1818. Capt. Henry Kater . . . . 389:37079 inches. 
£66.05 Gens At Ro Clarke ty \m)\s204 12 (8937043), | 
1883. Prof. Wm. A. Rogers. . . .- 39°37027 “ 
1885. Gen. C. B. Comstock . . . . 389°36985 “ 
The earliest standard of English weight of which we have any 
very definite knowledge is the mint pound of the Tower of London. 
It weighed 5,400 troy grains, and the coinage was regulated by it 
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