To spend the same amount of time in observing what happens when a brick is allowed to slide down a board or mercury is poured on a glass plate would be nonsense, and the writer of laboratory manuals feels himself driven to a verification, or study, as he may term it, of the quantitative laws. We thus too often find the student emulating Galileo in his discovery of the law of the pendulum. The absurdity of this attitude must be sufficiently obvious from the fact that in practise the student has always to be told what to discover and that it took the greatest men more than one laboratory exercise to find these laws originally.
It is at any rate true that the observation of physical phenomena for the purpose of mere acquaintance forms small part of any laboratory course, except in the more advanced parts of the subject, such, for example, as light and sound, and contrariwise those topics which demand such examination and acquaintance are commonly considered as too difficult of comprehension to be given to a beginner. From these considerations it must be evident that the usefulness of the physical laboratory can not be inferred from the benefits derived from the laboratory teaching of chemistry, but must be judged by a scale of values peculiarly its own. We have called physics an exact science. Now one of the uses of a physical laboratory is to make clear the meaning of that much misunderstood term 'exact.'
When Galileo was asked by the perplexed engineers why it was that water would not rise in their pumps to more than thirty feet he is said to have returned their question with another, 'Why does it rise at all?' To which they gave the current explanation, 'because nature abhors a vacuum.' 'Well then I suppose nature's abhorrence must cease at thirty feet' was the philosopher's doubtless knowing but evasive reply. That there is a limit to the elevation of liquids by atmospheric pressure and why is now understood by every educated person, but there are comparatively few who appreciate that exactness, like the schoolmen's horror vacui, ceases after a few significant figures.
The three fundamental magnitudes, time, length and mass, each possess some peculiar property in virtue of which they may be more accurately measured than almost any of the other physical magnitudes. Thus the length of the solar day is said to bear the ratio to the sidereal day of 1.00273791 to 1, an accuracy of one part in a hundred million.
The international kilogram has not been determined beyond 3/1,000 of a milligram, which implies an accuracy of one part in three hundred million.
The international meter has been measured in terms of the wave length of light to about one part in ten million, but such accuracy as that mentioned is attainable only in exceptional instances and enormously exceeds that within reach of ordinary careful work, which rarely extends to one part in ten thousand.