LETTERS TO BROTHER JOHN. 1 19 



it only visible at the extreme point of a long per- 

 spective. In short, he is timid, desponding, infirm 

 of purpose, imaginative, and incapable of continued 

 application. 



Such a man may be a poet, but not a mathe- 

 matician. 



On the contrary, when CONTRACTILITY is vi- 

 gorous, and the circulation consequently energetic, 

 the brain will be abundantly supplied with healthy 

 blood ; its nervous tissue firmly supported every- 

 where within the meshes of that tissue formed by 

 the interlacings of well-filled blood-vessels ; and its 

 SENSIBILITY, therefore, will be, in a corresponding 

 degree, obtuse. It requires a strong moral cause 

 to operate on the mind of such a man. " Trifles 

 light as air," have no power to excite, to irritate, or 

 in any way affect him : he is, consequently, bold, 

 patient, good-humoured, inflexible, unimaginative, 

 and capable of long-continued mental exertion*. 

 Such a man may become a great mathematician, 

 but never a poet. I think I could shew that all the 



* Why does exercising the memory strengthen the memory ? 

 Clearly, because it strengthens the brain by increasing the 

 vigour of the circulation through it. The use of the hammer 

 strengthens the blacksmith's arm in the same manner ; 

 viz. by increasing the vigour of the circulation through it ; 

 and consequently increasing also its contractility, that it, its 

 strength. 



