INTERMIXTURE OF EFFECTS. 



m 



■topped altogether ; it is clear that 

 the general proposition, though it 

 would be true under a certain hypo- 

 thesis, would not express the facts as 

 they actually occur. To accommodate 

 the expression of the law to the real 

 phenomena, we must say, not that 

 the object moves, but that it tends to 

 move, in the direction and with the 

 velocity specified. We might, indeed, 

 guard our expression in a different 

 mode, by saying that the body moves 

 in that manner unless prevented, or 

 except in so far as prevented, by some 

 counteracting cause, but the body 

 does not only move in that manner 

 unless counteracted ; it tends to move 

 in that manner even when counter- 

 acted ; it still exerts in the original 

 direction the same energy of move- 

 ment as if its first impulse had been 

 undisturbed, and produces, by that 

 energy, an exactly equivalent quantity 

 of effect. This is true even when the 

 force leaves the body aa it found it, in 

 a state of absolute rest ; as when we 

 attempt to raise a body of three tons 

 weight with a force equal to one ton. 

 For if, while we are applying this 

 force, wind or water or any other 

 agent supplies an additional force just 

 exceeding two tons, the body will be 

 raised ; thus proving that the force 

 we applied exerted its full effect by 

 neutralising an equivalent portion of 

 the weight which it was insufficient 

 altogether to overcome. And if while 

 we are exerting this force of one ton 

 upon the object in a direction con- 

 trary to that of gravity, it be put into 

 a scale and weighed, it will be found 

 to have lost a ton of its weight, or, in 

 other words, to press downwards with 

 a force only equal to the difference of 

 the two forces. 



These facts are correctly indicated 

 by the expression tendency. All laws 

 of causation, in consequence of their 

 liability to be counteracted, require 

 to be stated in words affirmative of 

 tendencies only, and not of actual re- 

 sults. In those sciences of causation 

 which have an accurate nomenclature, 

 there are special words which signify 



a tendency to the particular effect 

 with which the science is conversant; 

 thus pressure, in mechanics, is synony- 

 mous with tendency to motion, and 

 forces are not reasoned on as causing 

 actual motion, but as exerting pres- 

 sure. A similar improvement in ter- 

 minology would be very salutary in 

 many other branches of science. 



The habit of neglecting this neces- 

 sary element in the precise expres- 

 sion of the laws of nature has given 

 birth to the popular prejudice that all 

 general truths have exceptions ; and 

 much unmerited distrust has thence 

 accrued to the conclusions of science 

 when they have been submitted to 

 the judgment of minds insufficiently 

 disciplined and cultivated. The rough 

 generalisations suggested by common 

 observation usually have exceptions ; 

 but principles of science, or, in other 

 words, laws of causation, have not. 

 " What is thought to be an exception 

 to a principle," (to quote words used 

 on a different occasion,) "is always 

 some other and distinct principle 

 cutting into the former ; some other 

 force which impinges* against the 

 first force, and deflects it from its 

 direction. There are not a law and an 

 exception to that law, the law acting 

 in ninety-nine cases, and the excep- 

 tion in one. There are two laws, each 

 possibly acting in the whole hundred 

 cases, and bringing about a common 

 effect by their conjunct operation. 

 If the force which, being the less con- 

 spicuous of the two, is called the dis- 

 turbing force, prevails sufficiently over 

 the other force in some one case, to 

 constitute that case what is commonly 

 called an exception, the same disturb- 

 ing force probably acts as a modifying 

 cause in many other cases which no 

 one will call exceptions. 



" Thus if it were stated to be a law 

 of nature that all heavy bodies fall to 

 the ground, it would probably be said 



* It seetna hardly necessary to say that 

 the word impinge, as a general term to ex- 

 press collision of forces, is here used by a 

 figure of speech, and not as expressive of 

 any theory respecting the nature of force. 



