320 REPORT — 1844. 



a foot to a foot and a half in height. Its height gradually diminished, and 

 after a chase of one or two miles I lost it in the windings of the channel. 

 Such, in the month of August 1834-, was my ^rst chance interview with that 

 singular and beautiful phtenomenon which I have called the Wave of Trans- 

 lation, a name which it now very generally bears ; which I have since found 

 to be an important element in almost every case of fluid resistance, and as- 

 certained to be the type of that great moving elevation of the sea, which, wth 

 the regularity of a planet, ascends our rivers and rolls along our shores. 



To study minutely this phasnomenon with a view to determine accurately 

 its nature and laws, I have adopted other more convenient modes of produ- 

 cing it than that which 1 have just described, and have employed various 

 methods of observation. A description of these \yill probably assist me in 

 conveying just conceptions of the nature of this wave. 



Genesis of the Wave of the First Order. — For producing waves of the 

 first order on a small scale, I have found the following method sufficiently 

 convenient. A long narrow channel or box a foot wide, eight or nine inches 

 deep, and twenty or thirty feet long (Plate I. fig. 1.), is filled with water to 

 the height of say four inches. A flat board P (or plate of glass) is provided, 

 which fits the inside of the channel so as to form a division across the channel 

 where it is inserted. 



Genesis by InipuIsio7i or Force horizontally applied — Let this plate be in- 

 serted vertically in the water close to the end A, and being held in the verti- 

 cal position, be pressed forward slowly in the direction of X, care being taken 

 that it is kept vertical and parallel to the end. The water now displaced by 

 the plate P in its new position accunmlates on the front of the plane forming 

 a heap, which is kept there, being inclosed between the sides of the channel 

 and the impelling plate. The amount thus heaped up is plainly the volume 

 of water which has l)een removed by the advancing plane from the space left 

 vacant behind it, and if the impulse increase, the elevation of displaced 

 water will increase in the same quantity. When the water has reached the 

 height P,, let the velocity of impulsion be now gradually diminished as at P^, 

 until the plate is finally brought to rest as at P=, ; the height of the water 

 heaped on the front will diminish with the diminution of velocity as at P^, 

 and when brought to rest at P,, it will be on the original level. The total height 

 of the water does not however subside with the diminution of the impulsion, 

 the crest W^ retains the maximum height to which it had risen under the 

 pressure of the plane at P,, and moves horizontally forward ; and the smaller 

 elevation produced by the smaller pressure at P4 down to P^ moves forward 

 after Wj. This elevation of the liquid, having a crest., or summit, or ridge in 

 the centre of its length transverse to the side of the channel, continues to 

 move along the channel in the direction of the original impulsion ; from the 

 crest there extends forward a curved surface. Wo, forming the face of the 

 wore, and a similar surface, Wji', behind the crest is distinguished as its hack. 

 It is convenient to designate a as the origin, w as the end of the wave ; and 

 to designate the interval between a and tv, the length of the wave in the di- 

 rection of its transmission, its amplitude. 



The kind of motion required for generating this wave in the most perfect 

 way, that is, for producing a wave of given magnitude without at tiie same time 

 creating anv disturbance of a different kind in the water — this kind of motion 

 may be given by various mechanical contrivances, but I have found that the 

 dexterity of manipulation which experience bestows is perfectly sufficient for 

 ordinary experimental purposes. 



Genesis by a Colmnn nf Fluid. — This is a method of genesis, of considerable 

 value for various experimental purposes, especially useful when waves of nd 



