6o POPULAR SCIENCE MONTHLY. 



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 prac- 

 tise 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 de- 

 mand 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 labora- 

 tory 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 enor- 

 mously exceeds that within reach of ordinary careful work, which rarely 

 extends to one part in ten thousand. 



