37 



weights be applied. The danger which might arise from this fact 

 does not appear to have been at one time duly estimated ; and when 

 smaller engines were used than at present, and the axles were dif- 

 ferently cranked, the author thinks there is reason to believe that the 

 accidents which not unfrequently occurred with these engines (some 

 of them attended by fatal results) were due to this cause. The fact 

 seems first to have been brought prominently under the notice of 

 engineers by the experiments of Mr. George Heaton of Birmingham, 

 who caused a falsely-balanced wheel to roll round the periphery 

 of a circular table, by means of an axis fixed to a pivot in its centre, 

 and thereby exhibited the tendency to jump created by even a small 

 displacement of the centre of gravity. 



The analytical investigation in this paper shows how carefully the 

 crank should be counterbalanced to provide the requisite security 

 against the jumping of the wheel. It appears, that, assuming the 

 weight of an engine to be from 20 to 25 tons, and of a pair of six- 

 feet driving wheels from 2J to 3 tons, a displacement of the centre 

 of gravity of the wheel of about 3 inches from its centre would be 

 sufficient to cause it to jump at any instant when it attained a speed 

 of sixty miles an hour. 



A table is given in the paper of the displacements of the centre 

 of gravity necessary to produce jumps at different speeds. These 

 vary inversely as the squares of the speeds. 



Before a jump can take place, there must be a slip of the wheel, 

 or at least the wheel must cease to bite upon the rail ; and to this 

 cause, as well as to the reciprocating action of the two pistons, the 

 author considers may be due some portion of that fish-tail motion 

 which is familiar to railway travellers. The calculations show the 

 danger to be increased as the diameter of the driving wheel is di- 

 minished, and they are unfavourable to the use of light engines. 



March 20, 1851. 



SIR BENJAMIN C. BRODIE, Bart., Vice-President, in the Chair. 



A paper was in part read, entitled *' On the Anatomy and Phy- 

 siology of Salpa and Pyrosoma." By Thomas H. Huxley, Esq. 

 Communicated by Ed. Forbes, Esq., F.R.S. Received February 26, 

 1851. 



March 27, 1851. 

 SIR RODERICK I. MURCHISON, V.P., in the Chair. 



The reading of Mr. Huxley's paper " On the Anatomy and Phy- 

 siology of Salpa and Pyrosoma,^' commenced at the previous meet- 

 ing, was concluded. 



The object of the author in the present paper is to inquire into 

 the true nature of the singular phaenomena of reproduction in the 



