ON SHIP WA VES. 483 



trace a complete curve thus ABC and ADC ; 

 there is a perfect cusp in each curve at B and D 

 respectively, although it cannot be shown perfectly 

 in the drawing. Another formula, which need not 

 be reproduced here, gives a wave-height for every 

 point of those curves. Take alternate curves for 

 hollows and for crests ; and now in clay or plaster 

 of Paris mould a form corresponding with the 

 elevation due to the curve AB, plus the elevation 

 due to the curve BC, adding the two together; 

 thus you get for every point of your curves a 

 certain wave-height. With the assistance of Mr. 

 Maclean and Mr. Niblett the beautiful clay model 

 which is before you has been made, and it shows 

 the results of the theory constructed from actual 

 calculation. I will tell you how to construct the 

 angle of 19 28' made by each of those two straight 

 lines AB and AD with the direction of motion CA. 

 Draw a circle ; produce the diameter from one end 

 to a length equal to the diameter ; and from the 

 outer extremity of this projecting line draw two 

 tangents to the circle. Each of those tangent 

 lines makes an angle of 19 28' with the produced 



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