APPLICATIONS TO MECHANICS 101 



the waves of a rough sea. If the characteristic move- 

 ments of a boat in still water can be approximately 

 calculated beforehand, those on a rough sea can only 

 be ascertained by actual trial. Chronophotography 

 readily lends itself to researches of this kind. 



Experiments with floating objects, shaped more or 

 less like boats, show that it is easy to discover the 

 centre or the temporary centres of the rolling. By 

 placing a little model boat in front of a dark back- 

 ground, and by attaching bright objects to the mast, 

 a series of dotted lines is obtained by means of chrono- 

 photography, each of which represents one of the 

 successive phases of rolling, and indicates the position 

 of the mast ; but if these dots are joined up so as to 

 form continuous lines, and the latter are then produced 

 until they intersect below the level of the water, the 

 exact centre of oscillation can be determined for any 

 particular moment, provided, of course, that the points 

 of intersection are accurately obtained. If the floating 

 object be cylindrical in shape, the oscillation takes 

 place round the axis of the cylinder, but in other forms, 

 especially if the object is provided with a keel, the 

 oscillation takes place round centres which are con- 

 stantly changing. 



A celebrated French marine engineer assisted us in 

 making some researches on the rolling of ships under 

 more practical conditions ; the experiments were carried 

 out with small models which represented the commoner 

 types of ship. 



Similar researches may be carried out at the seaside 

 by fixing electric lights to the mast-heads of boats, 

 and by taking photographs of them during the dark- 

 ness of night. 



Vibrations of Metal Bridges. — M. Deslandres * has 



* Deslandres, "Action of Rhythmic Shocks upon Metal Bridges," 

 Annales des Fonts et Chaustees, Dec, 1892. 



