Page:Popular Science Monthly Volume 19.djvu/675

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ABOUT MEASURES OF LENGTH.

ing can be done from a distance of about eight metres (over twenty-five feet) from the balance. In the room where standards of length are compared, there is a comparator which cost about three thousand dollars, and another one is probably completed by this time, which is worth five thousand six hundred dollars.

It seems strange that the precise relation existing between the imperial standard yard and the metre of the Archives is not known. Nevertheless, it is true, for the two measures have never been directly compared. No equation can be assigned to them that is not likely to be erroneous by at least ·005 of an inch; but, in the year 1878, Parliament declared that the legal value of the metre, in inches, should be 39·37076.

It may be thought that so small an error as five one-thousandths of an inch, in a bar more than three feet long, is not worthy of serious consideration; but the fact is, that any error that can be detected by the most refined instrumental means is of great consequence, especially for scientific work. The error mentioned above will appear as a relatively large one, when we state with what accuracy minute measurements may be conducted. Professor Rogers considers that the error in comparing the length of two metre bars need not exceed the one-millionth part of a metre. In terms of an inch this error, expressed in figures, would be ·0000039. In testing the performance of his excellent comparator, he found the value of a centimetre, in terms of an inch, to be ·393707. The generally accepted value is ·393708, which indicates a truly wonderful degree of accuracy in the instrument.

Professor E. W. Morley has made some experiments to determine the probable error in micrometric measurements, and he has found that the errors of a single observer, under the particular conditions described, were very small. With a low-power object-glass, the probable error does not exceed thirty-nine ten-millionths of an inch. With a greater magnifying power he found it to be about thirty millionths of an inch. These errors, inconceivably small as they are, can be made appreciable by means of a microscope.

 
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ARE CEMETERIES UNHEALTHY?
By M. G. ROBINET.

IF the tomb is characteristic of humanity, as Vico has said, the cemetery, M. Pierre Lafitte remarks, is absolutely necessary to all human society. It not only furnishes a more or less hygienic method of disposing of the bodies of those who are no more—it is also a fundamental institution, in the sense that it is a symbol in no way arbi-