2 86 TEE POPULAR SCIENCE MONTHLY 



lish foot contains 304 mm., which is usually held to differ slightly from 

 that in vogue in the United States. Until there was a great deal of 

 national and international intercourse the need of some uniform stand- 

 ard of weights and measures was not seriously felt; consequently the 

 efforts of physicists in the seventeenth and eighteenth centuries did not 

 receive much encouragement. Absolute accuracy in matters of this 

 kind is unattainable^ but in practical affairs it is not particularly diflfi- 

 cult. What the term " accuracy " means to a maker of instruments of 

 precision is forcibly illustrated by an anecdote told of John A. Brashear, 

 of Pittsburgh. A prospective customer once asked him what it would 

 cost to have a bar of glass made that was absolutely straight. Mr. 

 Brashear would not promise absolute straightness, but was willing to 

 come as near as he could for two hundred thousand dollars. After 

 listening to a lecture on absolute accuracy by the renowned mechanician 

 the customer concluded that his needs would be supplied by a ruler 

 that would be correct to the one sixty-fourth of an inch and costing 

 about forty dollars. 



Physicists became convinced long ago that the only fixed standard 

 of linear measure is some portion of the earth's circumference. No in- 

 telligent Greek or Eoman from the time of Plato had any doubts about 

 the shape of the earth. But after the Bible had come to be recognized 

 as an authority in science as well as in doctrine the belief was gradu- 

 ally abandoned and various theories took the place of the true one until 

 the time of Copernicus. Archimedes, about 200 B.C. used an ingenious 

 argument to prove the sphericity of our planet. As water always seeks 

 the lowest level the ocean must be equally deep everywhere and the bot- 

 tom equally distant from a central point. As this is possible only in 

 the case of a sphere, the earth must be spheroidal in form. The first 

 attempt to calculate the circumference of the earth was made by the 

 celebrated savant Eratosthenes in the third century B.C. Observing 

 that the difference of latitude between two points in Egypt, Alexandria 

 and Syene, was 7° 12' and supposing them to be on the same meridian, 

 and having ascertained as best he could that they were about five thou- 

 sand stades apart, he reckoned this to be the fiftieth part of the earth's 

 circumference, which would accordingly be 250,000 stades. More than 

 a century later Poseidonius estimated the distance between Rhodes and 

 Alexandria, on the testimony of seamen, to be five thousand stades, or 

 one forty-eighth part of the circumference. Putting the value of the 

 stade at six hundred feet — authorities vary considerably on this point 

 — both estimates must be considered a remarkably close approximation 

 to the truth. 



In 1525 Fernel measured the distance between Paris and Amiens 

 with a wheel. Almost a century later Snellius discovered, or rather re- 

 discovered, trigonometry, which greatly simplified geodesy. By this 



