278 THE PRINCIPLES OF SCIENCE. [CHAP. 



senses for estimating the magnitudes of objects, or de- 

 tecting the degrees in which phenomena present them- 

 selves. " Things escape the senses," he says, " because the 

 object is not sufficient in quantity to strike the sense : as 

 all minute bodies ; because the percussion of the object is 

 too great to be endured by the senses : as the form of the 

 sun when looking directly at it in mid-day ; because the 

 time is not proportionate to actuate the sense: as the 

 motion of a bullet in the air, or the quick circular motion 

 of a firebrand, which are too fast, or the hour-hand of 

 a common clock, which is too slow ; from the distance 

 of the object as to place : as the size of the celestial 

 bodies, and the size and nature of all distant bodies; 

 from prepossession by another object : as one powerful 

 smell renders other smells in the same room imper- 

 ceptible ; from the interruption of interposing bodies : 

 as the internal parts of animals ; and because the object 

 is unfit to make an impression upon the sense : as the 

 air or the invisible and untangible spirit which is in- 

 cluded in every living body." 



Complexity of Quantitative Questions. 



One remark which we may well make in entering 

 upon quantitative questions, has regard to the great variety 

 and extent of phenomena presented to our notice. So 

 long as we deal only with a simply logical question, that 

 question is merely, Does a certain event happen ? or, Does 

 a certain object exist ? No sooner do we regard the event 

 or object as capable of more and less, than the question 

 branches out into many. We must now ask, How much 

 is it compared with its cause ? Does it change when the 

 amount of the cause changes ? If so, does it change in 

 the same or opposite direction ? Is the change in simple 

 proportion to that of the cause ? If not, what more com- 

 plex law of connection holds true ? This law determined 

 satisfactorily in one series of circumstances may be varied 

 under new conditions, and the most complex relations of 

 several quantities may ultimately be established. 



In every question of physical science there is thus a 

 series of steps the first one or two of which are usually 

 n?ade with ease while the succeeding ones demand more 



