168 DISCOURSE ON THE STUDY 



great number of facts led Galileo to conclude that 

 the accelerating power of gravity is the same on all 

 sorts of bodies, and on great and small masses indif- 

 ferently ; and this he exemplified by letting bodies 

 of very different natures and weights fall at the 

 same instant from a high tower, when it was ob- 

 served that they struck the ground at the same 

 moment, abating a certain trifling difference, due, as 

 he justly believed it to be, to the greater propor- 

 tional resistance of the air to light than to heavy 

 bodies. The experiment could not, at that time, 

 be fairly tried with extremely light substances, such 

 as cork, feathers, cotton, &c. because of the great re- 

 sistance experienced by these in their fall ; no means 

 being then known of removing this cause of disturb- 

 ance. It was not, therefore, till after the invention 

 of the air-pump that this law could be put to the 

 severe test of an extreme case. A guinea and a 

 downy feather were let drop at once from the upper 

 part of a tall exhausted glass, and struck the bot- 

 tom at the same moment. Let any one make the 

 trial in the air, and he will perceive the force of an 

 extreme case. 



(178.) In the verification of a law whose expression 

 is quantitative, not only must its generality be esta- 

 blished by the trial of it in as various circumstances 

 as possible, but every such trial must be one of pre- 

 cise measurement. And in such cases the means 

 taken for subjecting it to trial ought to be so de- 

 vised as to repeat and multiply a great number of 

 times any deviation (if any exist) ; so that, let it be 

 ever so small, it shall at last become sensible. 



(179.) For instance, let the law to be verified 



