MRS. SOMERVILLE. 7 



in fact, that neither Mrs. Somerville nor Sir John 

 Herschel thoroughly apprehended the difficulty of con- 

 veying to the general reader clear ideas respecting even 

 the elements of the subjects they severally endeavoured 

 to expound. But I feel bound to add that Mis. 

 Somerville's failure, inevitable from the very nature of 

 her task, would in any case have been brought about 

 by the manner in which the task was accomplished. 

 It will presently be seen that, in saying this, I am, in 

 fact, touching on the most remarkable and distin- 

 guishing quality of Mrs. Somerville's mind. 



There are two essential requisites in a treatise in- 

 tended to introduce a difficult subject to general readers. 

 First, there must be a clear apprehension of the position 

 of such readers, of what they can and of what they 

 cannot understand, and of the form in which what is 

 written for them may most usefully be presented. It 

 is not too much to say that if just ideas had been en- 

 tertained by Mrs. Somerville on this point, the attempt 

 to present the Mechanism of the Heavens in a popular 

 form would never have been made. But, secondly, it is 

 essential that in any work of the kind each statement 

 each sentence, in fact should be presented in terms 

 so precise as to be absolutely unmistakable. This is 

 not so necessary in advanced treatises indeed, it is 

 too well known how large a proportion of our works on 

 advanced science are wanting in strict precision of 

 expression. But it is absolutely necessary in works 

 intended to popularise science. It is a somewhat 

 remarkable circumstance that in the Mechanism of the 



