NO. 16.] APPARATUS AND METHODS. 59 



Fig. 2. When it arrived at the other end of the tank, the hoat glided into 

 a sort of slip to avoid thrusts and agitations in the water-layers. 



The boat-models were made of solid wood and loaded to the weight stated 

 for each of them. The screw-propeller and rudder-post (but not the rudder) 

 were omitted, as well as most of the vessels body above the water-line. The 

 small Fram-model (1 : 200) used in the small tank, was of painted fir ; the small 

 Fram-model (1 : 200) as well as all the other models used in the large tank, 

 were of polished mahogany. The shape of each of them is described below in 

 section F of this chapter. The towing-thread was fixed to the boat, to a sort 

 of bow-sprit, so as always to be nearly horizontal. If the boat moved freely, 

 it made greater and greater sheers and finally struck the side of the tank. 

 To avoid this inconvenience, a little fork fixed to the stem of the boat was 

 made to glide along a fine silvered copper-wire, stretched tight along the mid- 

 line of the tank. The friction against the wire increased with the velocity of 

 the boat and was between 5 and 20 cgr. 



When the boat-model was in motion, it created small oscillations in the 

 water-level, and these were measured by particular experiments. As level- 

 gauge a disc, 5 cm. in diameter, was used. It was suspended from the shorter 

 end of a light balance, so as to adhere to the water-surface, which it lifted a 

 little above its free level; by means of levers, the vertical motion of the disc 

 was communicated 50-fold to a pointer which allowed the variations of level 

 to be recorded with an accuracy of 0*02 mm. The apparatus was fastened in 

 a suitable place to the upper beam of the tank. The disc was very insensible 

 to the horizontal motions in the water, provoked by the boat passing — prob- 

 ably because it was raised above the free level of the surface. Horizontally 

 it never moved so much as half a mm., which would have caused an error of 

 0001 mm., only, in the observation of the level. When all waves in the water 

 had disappeared and the level-gauge was adjusted with its pointer at zero, the 

 boat-model was unhooked. As the boat was moving along the tank, a free- 

 hand curve on squared paper representing the subsequent indications of the 

 level-gauge as ordinates with the corresponding positions of the boat as ab- 

 scissae was drawn by me. By a second experiment the curve was completed 

 and, if necessary, corrected. Provided the motion can be considered as sta- 

 tionary, these curves, drawn on a suitable scale, as well represent the profile 

 of water-level following in an invariable position relative to the boat. 



