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The progress of metrology has led, as a consequence, to corresponding progress in nearly all physical measurements, and particularly in the measure of natural constants. Among these, the constant of gravitation occupies a position quite apart from the importance and simplicity of the physical law which defines it, as well as by its generality.
On that occasion , Mr John Quincy Adams produced a most elaborate report to Congress, containing an immense amount of information on the subject of metrology. He found great fault with the French nomenclature, so puzzling to the unlearned.
If decimal arithmetic is incompetent to give the dimensions of most artificial forms, the square and the cube, still more incompetent is it to give the circumference, the area, and the contents of the circle and the sphere. And once more: 'The new metrology of France, after trying the principle of decimal division in its almost universal application, has been compelled to renounce it for all the measures of astronomy, geography, navigation, time, the circle, and the sphere; to modify it even for superficial and cubical linear measure. The conclusion of the Americans was, that it was better to continue the use of the system of weights and measures inherited from the father-land.
Nickel steels present the most curious properties, and I have already pointed out the paramount importance of one of them, hardly capable of perceptible dilatation, for its application to metrology and chronometry.
Metrology vouches for the hundredth of a milligramme in a kilogramme; that is to say, that it estimates the hundred-millionth part of the magnitude studied. We may as in the case of the lengths ask ourselves whether this already admirable precision can be surpassed; and progress would seem likely to be slow, for difficulties singularly increase when we get to such small quantities.
If all measurable quantities, as was long thought possible, could be reduced to the magnitudes of mechanics, metrology would thus be occupied with the essential elements entering into all phenomena, and might legitimately claim the highest rank in science.
Persons are still to be met with who seem to have some illusions on this point, and who see in the doctrine of the dimensions of the units a doctrine of general physics, while it is, to say truth, only a doctrine of metrology.
At the present day we designate more peculiarly by the name of metrology that part of the science of measurements which devotes itself specially to the determining of the prototypes representing the fundamental units of dimension and mass, and of the standards of the first order which are derived from them.
It is used for instance in the division of the circle into degrees, minutes, and seconds, in the division of the year into months, and of the day into hours and their fractions. This convenient, exact, and highly developed system of arithmetic and metrology enabled the Chaldæans to make good use of their observations, and to extract from them a connected astronomical doctrine.
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