Page:Popular Science Monthly Volume 18.djvu/384

370 researches that the whole scale be simply a millimetre one, and care only be taken to have the millimetre graduation extremely accurate.

The dividing of the tube so that an equal volume of mercury may occupy the same number of degrees at the various parts of the tube is called the calibration of a thermometer, and on the perfection of this work, if it is attempted at all, largely depends the value of the thermometer. As Pernet has remarked, the labor of determining the errors of a thermometer is much increased by having to determine the errors the maker has introduced in the imperfect calibration of its scale. In observations not requiring an accuracy beyond 0·1° F., it it might be quite safely left to the skill of a reputable maker to free the instrument from errors of this kind. It is accomplished by detaching a small portion of the column and measuring its length at different, and usually consecutive, parts of the tube. Obviously from these results may be computed the value of 1° at successive parts of the thermometer scale, in terms of the dividing engine used by the maker.

The precision attained in the calibration of standards when the greatest care is exercised is surprising; thus, in the three Kew standards of the Yale Observatory, the maximum sum of the errors depending on imperfect calibration is very nearly 0.01° in each of them.

Supposing that several thermometers, by different and equally skillful makers, have been prepared with the greatest care, it is found in comparing them that they differ sensibly among themselves, owing to the difference in the glass used in their construction, their varying sensitiveness to the slight changes caused by the circulation of the water in which they are immersed, and a variety of less obvious causes. It becomes necessary, therefore, that some definite construction of the mercurial standard thermometer should be adopted, and the standard chosen by the Yale Observatory is defined upon the certificates issued with standards compared, as follows:

"The theoretical mercurial standard thermometer to which this instrument has been referred, is graduated by equal volumes upon a glass stem of the same dimensions and chemical constitution as the Kew standards 578 and 584. The permanent freezing-point is determined, by an exposure of not less than forty-eight hours to melting ice, supposing the temperature of the standard has not been greater than 25° Cent. $$=$$ 77° Fahr. during the preceding six months. The boiling-point is determined from the temperature of the steam of pure water at a barometric pressure of 760 mm. $$=$$ 29·922 inches (reduced to 0° Cent.) at the level of the sea and in the latitude of 45°."

This standard has its 0° and 100° C. identical with the standard of the International Commission of Weights and Measures, and the physicists generally have agreed upon the pressure and latitude given as the most advisable. It is practically coincident with a pressure of 29·905 inches in the latitude of London, and at the sea-level—the