414 INDUCTION. 



fathers. On the other hand, the pressure which forces 

 up the mercury in an exhausted tube must be con- 

 tinued in order to sustain it in the tube. This (it may 

 be replied,) is because another force is acting without 

 intermission, the force of gravity, which would restore 

 it to its level, unless counterpoised by a force equally 

 constant. But again ; a tight bandage causes pain, 

 which pain will sometimes go off as soon as the ban- 

 dage is removed. The illumination which the sun 

 diffuses over the earth ceases when the sun goes 

 down. 



The solution of these difficulties will be found in 

 a very simple distinction. The conditions which are 

 necessary for the first production of a phenomenon, 

 are occasionally also necessary for its continuance ; 

 but more commonly its continuance requires no con- 

 dition except negative ones. Most things, once pro- 

 duced, continue as they are, until something changes 

 or destroys them ; but some require the permanent 

 presence of the agencies which produced them at first. 

 These may, if we please, be considered as instanta- 

 neous phenomena, requiring to be renewed at each 

 instant by the cause by which they were at first gene- 

 rated. Accordingly, the illumination of any given 

 point of space has always been looked upon as an 

 instantaneous fact, which perishes and is perpetually 

 renewed as long as the necessary conditions subsist. 

 If we adopt this language we are enabled to avoid 

 admitting that the continuance of the cause is ever 

 required to maintain the effect. We may say, it is 

 not required to maintain but to reproduce the effect, 

 or else to counteract some force tending to destroy it. 

 And this may be a convenient phraseology. But it is 

 only a phraseology. The fact remains, that in some 

 cases (though these are a minority,) the continuance 



