302 SECTION MECHANICS. 



Having told yon how I make the models, I must next tell you how 

 I test them. That is as important a part of the business as making 

 them. I will ask you for the present to take for granted, what I will 

 endeavour to explain presently in detail, that there is ? true scale of 

 comparison between the curve of resistance for a model, and that 

 for a ship similar to the model. Meantime, I wi'.l describe how we 

 ascertain the curve of resistance for the model. Our models vary 

 from six to sixteen feet in length, from eighteen inches to t\vo feet six 

 inches in breadth, and weigh from 200 pounds up to 600 pounds, 700 

 pounds, and 800 pounds. They are not puny little models. 1 believe 

 even much smaller models, if correctly interpreted would give valuable 

 results, but these are irreproachably large models. The apparatus for 

 testing consists of the place in which the testing is done, and the 

 testing apparatus itself. The place in which it is done consists of a 

 tank or water space, 280 feet in length, ten feet deep, and thirty-six 

 feet wide, roofed over throughout. Compared with the length of the 

 models we use, the run we are able to give them in testing, is 

 nearly equivalent to the measured mile for the largest ships in the 

 navy, and the depth of the water is about equivalent to the depth 

 of the British Channel, so that we proceed on a large scale of 

 operations, and one by which we have no fear of encountering an 

 adventitious resistance due to the formation of shallow-water waves. 

 The apparatus is arranged as follows : 



Just above the surface of the water, throughout the length of the 

 water space, is laid a railway, perfectly level and firmly suspended 

 from theM'oof of the building without the interposition of any inter- 

 vening columns near the track of the model, so that there is nothing 

 to obstruct the motion of the water ; and a dynamometrical carriage 

 runs on th's railway. In the dynamometric carriage there is a 

 form of apparatus, well understood by many of you, a revolving 

 cylinder, the circumferential motion of which represents, to scale, 

 the progressive motion of the railway carriage. The scale is exactly 

 one-fifth of an inch to a foot, so that every foot the railway carriage 

 moves, is represented by one-fifth of an inch of circumferential 

 motion of the cylinder. There is under the railway carriage, a spring 

 balance, and to that the model is hooked or harnessed. The only 

 force which is applied to the model is through this spring balance, 



