INTERMIXTURE OF EFFECTS. 31 9 



that it te^ids 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 maimer 

 even when counteracted ; it still exerts, in the original direction, the same 

 energy of movement as if its first impulse had been undisturbed, and pro- 

 duces, by that energy, an exactly equivalent quantity of effect. This is 

 true even when the force leaves the body as it found it, in a state of abso- 

 lute 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 ex- 

 erted its full effect, by neutralizing an equivalent poi'tion 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 contrary 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 downward 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 results. 

 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 2^'>'^ssure, in mechanics, is synony- 

 mous with tendency to motion, and forces are not reasoned on as causing 

 actual motion, but as exerting pressure. A similar improvement in termi- 

 nology would be very salutary in many other branches of science. 



The habit of neglecting this necessary element in the precise expression 

 of the laws of nature, has given birth to the popular prejudice that all gen- 

 eral truths have exceptions ; and much unmerited distrust has thence ac- 

 crued to the conclusions of science, when they have been submitted to the 

 judgment of minds insufficiently discipUned and cultivated. The rough 

 generalizations 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 cut- 

 ting 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 excep- 

 tion to that law, the law acting in ninety-nine cases, and the exception 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 conspicuous of the two, is called the disturbinr/ 

 force, prevails sufficiently over the other force in some one case, to consti- 

 tute that case what is commonly called an exception, the same disturbing 

 force probably acts as a modifying cause in many other cases Avhich 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 that the resistance of the atmos- 

 phere, which prevents a balloon from falling, constitutes the balloon an ex- 



* It seems hai'dly necessary to say that the word impinge, as a general term t 

 lision of forces, is here used by a figure of speech, and not as expressive of anjjfr 

 ing the nature of force. ' 



