ON THE ACTION OF WAVES AND CURRENTS. 329 



and that consistently with this the generators should be as short as 

 possible. This tide in tank A required that the generator should displace 

 10 cubic feet, and as the greatest rise and fall that could be conveniently 

 obtained for the end of the generator was IG", giving a mean rise of 8", 

 the area required was 15 square feet. 



A period of 30 seconds was adopted for tank A as the shortest period 

 likely to be required, and the gearing arranged accordingly. With this 

 period, and a 2" tide, the horizontal scale would be 1 in 20,000 of that of 

 a tank with a 30-foot tide, and a period of 12 hours 20 minutes. So that 

 the 12-foot pan would represent 45 miles. 



Provision was made for the production of waves with periods ^oth 

 the tidal period. 



Provision was also made for the introduction of land water into the 

 tank at any point that might be required ; also for scumming the water 

 by an adjustable weir, Avhich would serve to keep the level of low water 

 constant, water being supplied into the generator when no land water 

 was required. 



The drawings (Plate T.) show the tanks and apparatus as they have 

 been constructed. The pans and tide generators are of pine-boards 

 fastened with screws. The former rest in a fixed cradle formed by six 

 legs with cross-bearers, bottom ties, and braces. The floor boards of the 

 pan are screwed to the cross-bearers, but are left free to expand, the joints 

 being made with marine glue, after the manner of the decks of ships. 

 The sides are screwed to the floor only ; they receive lateral support 

 against the pressure of the water from the prolongations of the legs up- 

 wards. The pans are lined with calico saturated with marine glue, and 

 put down with hot irons, then covered Avith a coat of paraffin. The pans 

 of the generators are constructed in the same way as the others, only 

 instead of the cross-bearers being attached to legs they are suspended 

 from two side levers which are supported on cast-iron knife-edges resting 

 in cast-iron grooves on the top of the legs at the end of the pan. These 

 knife-edges are at the exact level of the top of the floor of the pan, and 

 in line with the joint in the floor between the pan and the generator, so 

 that there is no opening and closing of this joint. This joint is, how- 

 ever, covered with indiarubber, which extends up the sides, and by 

 stretching allows for the opening and closing of these joints. 



In tank A these side levers extend 4 feet along the sides of the pan, 

 beyond the joint, and to their ends is attached a large box for holding 

 balance weights. These weights are considerably below the knife-edges, 

 and consequently their moment diminishes as the box descends, i.e. as 

 the tide rises, but this diminution by no means compensates the diminu- 

 tion of the water in the generator. 



If, therefore, sufficient weight were put into the box to balance tho 

 generator when the tide is low, it would much overbalance it when the 

 tide is high. To meet this the weights in the box are used mainly to 

 balance the dead weight of the generator, which requires about 300 lbs. 

 and a varying balance is arranged for the water. 



This varying balance consists, in tank A, of a cast-iron cylinder of 

 500 lbs. weight suspended by links from the side levers across under the 

 tank. The cylinder is also suspended by two links from the frame, and 

 this second suspension is so arranged that when the generator is down 

 the links from the levers are vertical, and when the generator is up they 

 are horizontal. In this way a varying balance is obtained, which as fur 



