126 BROWN : 



millionth of an inch. These statements the writer accepts in 

 childlike confidence, but of his own knowledge he knows 

 nothing of them. 



The standard of length, then, being a known portion of a 

 length which beats seconds, depends upon the standard of 

 time. 



The old, original standard of weight was, as in the case of 

 length, the grain of dried barley. It varied, of course, but 

 gradually took a value. 



To get a fixed, recoverable standard, a cubic inch of rain 

 water was weighed under recorded circumstances — latitude of 

 London — sea level — in vacuo — 62° Fahrenheit. In short, 

 precisely the same as in the matter of the pendulum. The 

 weight was 252.722 times that of the grain in use. 



Which means that when the weight of a cubic inch of 

 water, as above, is divided into two hundred and fifty-two 

 thousand seven hundred and twenty-two equal parts, one 

 thousand such parts is a grain. 



Seven thousand grains, that is, seven million such parts, 

 were called an Avoirdupois pound, and the pound was suit- 

 ably subdivided. The standard thus fixed is recoverable if 

 lost in the vicissitudes of things. 



The measure of weight depends upon the measure of 

 length, which depends upon the measure of time, whose unit 

 is one second. 



Now, what is a second ? 



Everything else left out of the question, the earth revolves 

 once on its axis in an invariable space of time, marked bj'^ 

 the return of a star to the meridian. This is called a mean 

 sidereal revolution of the earth. The word is " invariable," 

 and there is as yet nothing to show that we may not under- 

 stand the word in its fullest sense. This space of time is 

 divided into twenty-four hours, each hour into sixty minutes, 

 each minute into sixty seconds ; in all, 86,400 seconds. The 

 second, then, is the eighty-six thousand four hundredth part 

 of the earth's mean diurnal, sidereal revolution, and it is an 



