and their Action on Floating Bodies. 95 



risks there must be a certain degree of stability which com- 

 bines the maximum of safety. It would, perhaps, be small 

 consolation for a man to know he had assured himself from 

 the danger of being capsized by an extra liability, of going 

 straight down, and it may fairly be useful to apprehend the 

 condition that a safe ship is one which partially opposes the 

 waves, and partially evades them by obeying them. 



In like manner a very steady ship is not necessarily the 

 most comfortable one. The frequency with which the seas 

 must visit her decks, and the constant creaking of her frame 

 by the enormous moving force of water which she opposes 

 are considerations in a long voyage. Whether these or 

 greater freedom of motion on the waves be the less objec- 

 tionable is of course matter for individual judgment. 



It is certain, however, that very excessive steadiness will 

 never be attained ; the magnitude of ocean waves being 

 too great in comparison with the possible size of ships to 

 render it feasible. For instance, a wave only ten feet in 

 height (and the surface of mid-ocean, in calm or storm is 

 seldom if ever free from rollers of this magnitude) has a 

 breadth of never less than thirty feet ; so that we may 

 easily perceive the huge effect which the force of buoyancy 

 of such a wave must exert in shifting from one side to 

 another, even of a vessel of fifty feet beam. The largest vessel 

 yet constructed, the Great Eastern, is a notable example of 

 this remark. She follows the waves heavily in a sea-way. 



The investigations herein will tend to show that the 

 oscillation of a vessel on waves is the resultant of three 

 simultaneous oscillations. T think, therefore, that the 

 natural period of rolling of vessels when heeled over in 

 smooth water, which seems to be the basis of the extensive 

 experiments in ship-building now being made in England, 

 and which we are told other governments are closely watch- 

 ing, will be found to be at variance with the natural period 

 at sea. A more practical method would appear to be to deter- 

 mine by a course of wide-spread observations obtained in 

 self-reo-isterino; instruments, the actual movements of vessels 

 during their voyages. For this purpose an instrument has 

 been devised, by the assistance of which it is hoped also to 

 establish definitely the mean elements of ocean waves ; but 

 as its action depends upon what I have called the principle 

 of independent inertia, a somewhat abstruse subject, I will, 

 with your permission, reserve its consideration for another 

 paper. 



