CHAP. X.] SCIENTIFIC WORK. 289 



thought of my paper, it was very wrong ; for I do not think 

 any one should be called upon for the expression of their 

 thoughts before they are prepared, and wish to give them. 

 I have often enough to decline giving an opinion because 

 my mind is not ready to come to a conclusion, or does not 

 wish to be committed to a view that may by further con- 

 sideration be changed. But having received your last 

 letter, I am exceedingly grateful to you for it, and rejoice 

 that my forgetfulness of having sent the former paper 

 on conservation has brought about such a result. Your 

 letter is to me the first intercommunication on the subject 

 with one of your mode and habit of thinking. It will do me 

 much good, and I shall read and meditate it again and again. 

 I daresay I have myself greatly to blame for the vague 

 use of expressive words. I perceive that I do not use the 

 word " force " as you define it, " the tendency of a body to- 

 pass from one place to another." What I mean by the 

 word is the source or sources of all possible actions of the 

 particles or materials of the universe; these being often 

 called the powers of nature when spoken of in respect of the 

 different manners in which their effects are shown. In a 

 paper which I have received at the moment from the Phil. 

 Mag., by Dr. Woods, they were called the " forces, such as 

 electricity, heat, etc." In this way I have used the word 

 " force " in the description of gravity which I have given as 

 that expressing the received idea of its nature and source ; 

 and such of my remarks as express an opinion or are critical 

 apply only to that sense of it. You may remember I speak 

 to labourers like myself, experimentalists on force generally, 

 -who receive that description of gravity as a physical truth, 

 and believe that it expresses all, and no more than all, that 

 concerns the nature and locality of the power. To these it 

 limits the formation of their ideas, and the direction of their 

 exertions, and to these I have endeavoured to speak ; show- 

 ing how such a thought, if accepted, pledged them to a very 

 limited and probably erroneous view of the cause of the 

 force, and to ask them to consider whether they should not 

 look (for a time at least) to a source in part external to- 

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