Professor Forbes's Address to the British Associaturn. 253 



are these elements subject to abrupt and capricious changes, 

 which Baron Humboldt has termed magnetic storms ; but gra- 

 dual and progressive variations are undergone at different hours 

 of the day, at different seasons of the year, and throughout 

 longer periods, which may even perhaps bear a comparison with 

 the sublime cycles of Astronomy. 



Natural History forms a more prominent subject in this volume 

 than in the last, though the reports of Professor Lindley " on 

 the principal questions at present debated in the Philosophy of 

 Botany ;*" and of Dr Charles Henry " on the Philosophy of the 

 Nervous System,'*"' refer only to particular departments of widely 

 extended subjects, which are again to be resumed in more ge- 

 neral reports, undertaken for the present meeting, — that by Mr 

 Bentham, on Systematic Botany, and by Dr Clarke of Cam- 

 bridge, on Physiology in general. We cannot but remark with 

 pleasure, that one of the points for inquiry particularly insisted 

 on by Professor I^indley, that of the influence of the chemical 

 nature of soils, and of the excretions of plants, was taken up at 

 an early period of the existence of the Association, by one of 

 its most zealous supporters, Dr Daubeny ; and that, in refe- 

 rence to the review by Dr Henry of the labours of European 

 physiologists, we may quote, as a national honour, the discove- 

 ries of our distinguished Associate, Sir Charles Bell. 



On the general connexion and occasional apparent opposition 

 of Theory and Practice, I would refer to some very pertinent re- 

 marks in the Address of Mr Whewell, at the last meeting. The 

 importance of carrying on both simultaneously and independ- 

 ently, and of looking to our increased knowledge 'of both as the 

 only sure means of ultimately reconciling discrepancies, has been 

 manifested, by the desire of the Council of the Association to 

 procure two distinct reports on the Theory and Practice of Hy- 

 draulics, which have been drawn up with remarkable perspicuity, 

 and within a small compass, by Mr Challis and Mr Rennie. 

 Both of these gentlemen have shewn their zeal in the objects of 

 the Association, by promising to continue theirvaluable labours. 

 Mr Rennie, on that part of his subject which relates to the mo- 

 tion of fluids in open channels, and Mr Challis on some of those 

 exceedingly interesting branches of theory altogether modern, 



VOL. XVII. NO. XXXIV.— OCTOBER 1834 S 



