WEIGHTvCi AND MKASITRES. 1 25 



was manufactured with even- possible care and skill, and 

 without regard to cost, so that it should, under certain 

 recorded and easily reproduced circumstances, measure what 

 was to be called a yard. In short, a standard yard. One- 

 third of this was to be called a foot ; and one-twelfth of this 

 was to be called an inch. 



But the standard might be lost in the vicissitudes of things. 

 So to make it recoverable the length of a pendulum vibrating 

 seconds in vacuo — 62° Fahrenheit — latitude of London — at 

 the sea level — was determined. The standard inch was con- 

 tained in this pendulum thirty-nine times and thirteen hundred 

 and ninety-three ten thousandths of a time (nearly). That is 

 to say, the pendulum was 39.1393 (nearly) inches long. Now 

 set a sum in Rule of Three : — 



39-1393 : I ^ I : Ans. .02555 



That is to say, further, that the pendulum length was divided 

 into one hundred thousand equal parts, and two thousand 

 five hundred and fifty-five such parts were taken for an inch. 



Twelve times the inch is the foot, three thousand and 

 sixty-six ten thousandths of the pendulum length ; and three 

 times the foot is the yard, or nine thousand one hundred and 

 ninety -eight ten thousandths of the pendulum length. 



And thus the original unit is recoverable. 



It requires fairh' good eyesight to read the hundredth part 

 of an inch on the scale, and the hundredth part is divided 

 into ten thousand parts again to get the millionth part. But 

 the magnifying power of optics and the excessively minute 

 subdivision obtainable by delicate machinery, together with 

 the appliance called the vernier, are made substitutes for 

 increased power of perception. Accordingly I am informed, 

 generally, that the Board of Trade has instruments which 

 deal readily with the ten thousandth of an inch, and which 

 can read off the one hundred thousandth. There are also, it 

 is said, instruments — requiring, however, special precautions 

 and orent skill in the using — which will take account of the 



