PREFACE. ix 



communicate the results of their observations to the 

 world without waiting for a period (never perhaps to 

 arrive) when they may themselves have the credit of 

 completing and perfecting them. It is better, whilst 

 the freshness of recollection is undimmed, and the mind 

 is in a position to draw a correct parallel between older 

 and quite recent observations, and thus to give our 

 descriptions the necessary development, to throw into 

 them whatever there may be of value in our peculiar 

 experience or habits of investigation, and frankly to 

 invite our fellow-labourers to do in our stead what we 

 would gladly have done but for some inevitable want 

 of health, leisure, or opportunity. Were this course 

 more generally followed in the sciences, both of obser- 

 vation and experiment, I am persuaded that by grasping 

 at less we should attain more even in personal repu- 

 tation and should unquestionably advance the interests 

 of knowledge." 



Norfolk Crescent, Bath, 

 November, 1854. 



