REPORTS ON SPECIAL BRANCHES OF SCIENCE. 795 



state of a science- has for its main aim to enumerate the various authors and 

 to point out their relative weight, and this has been very well done in several 

 British Association Eeports, some of which are nearly as old as the British 

 Association. 



There are some branches of science whose position with respect to the 

 public, or else to the educational interest, is such that treatises or text-books 

 can be published on commercial principles, either as books to be read by the 

 free public, or to be got up by the school public. 



There is little encouragement, however, for a scientific man to write a 

 treatise so long as he can, with much less trouble, produce an original memoir, 

 which will be much more readily received by a learned society than the treatise 

 would have been by a publisher. 



The systematisation of science is therefore carried on under difficulties 

 when left to itself; and I think that the experience of the British Association 

 warrants the belief that its action in asking men of science to furnish reports 

 has conferred benefits on science which would not otherwise have accrued to it. 



There are so many valuable reports in the published volumes that I shall 

 indicate only a few, the selection being founded on the direction of my own work 

 rather than on any less arbitrary principle. 



First, when a branch of science contains abstruse calculations as well as 

 interesting experiments, it is desirable that those who cultivate the experimental 

 side should be conscious that certain things have been done by the mathema- 

 ticians. The matter to be reported on in this case is not voluminous, but it is 

 hard reading, and those who are not experts require a guide. 



Thus, Professor Challis in 1834 gave a most useful report on the mathe- 

 matical investigations by Young, Laplace, Poisson, and Gauss on Capillary 

 Attraction, and Professor Stokes in 1862 reports on Theories of Double Re- 

 fraction. This report may, indeed, be accepted as an instalment of the treatises 

 which, if the desire of the scientific world were law, Professor Stokes would 

 long ago have written. It is meant, no doubt, as a guide to other men's 

 writings, but it is intelligible in itself without reference to those writings. Such 

 a report is a full justification of the existence of the British Association, if it had 

 done nothing else. 



Another type of report is that of Professor Cay ley on Dynamics (1857 

 and 1862). This seems intended rather as a guide in reading the original 

 authors than as a self-interpreting document, though, of course, besides the 



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