NO. IB.] QUALITATIVE RESULTS. 67 



but in Fig. 1 the boat is drawn by these 3 gr. at a velocity of 9 - 5 cm. per 

 second only, whereas in Fig. 2 the boat has from the beginning a higher 

 velocity, and is steadily towed at a speed of 27 cm. per second. At the lower 

 velocity the boat pushes a mass of water before her stem, and at the stern 

 she provokes a wave-hollow; her resistance is in consequence, in creased just 

 as if she constantly had to rise on an incline. She is then "in dead- 

 water". At the higher velocity, on the other hand, the boat moves on the 

 top of the low hillock of water, which she provokes, and she consequently 

 moves on a nearly horizontal surface. The analogy with the phenomena 

 in shallow canals, described by Scott Russell (see pp. 38 — 39) is apparent 

 (compare Figs. 1 and 2, PL IX, with Figs. 6 and 7, p. 39). But there is the 

 difference that the waves illustrated in PL IX are not free waves in the water- 

 surface, but an effect of the boundary-waves below, arising at a much slower 

 velocity than the wave-phenomena in shallow water illustrated on p. 39. 



The energy which is represented by the disturbance of level, is, at the 

 lower velocity, constantly left behind as wave-motion; and the consequent loss 

 is refunded by the work done by the boat in pressing into the wall of water 

 before her. At the higher velocity, there is very little energy left behind, and 

 consequently very little work necessary for refunding the loss. 



A disturbance of the water-surface similar to that in the case of "dead- 

 water", takes place also in homogenous water (see curves 1 and 2, Fig. 3, 

 PL IX), but at much higher velocities than in dead-water. It will be shown in 

 section F, that the pressure-resultant influencing the boat-model in consequence 

 of this disturbance of water-level, makes an important part of the resistance, 

 in homogenous water as well as in "dead-water". In the latter case by far the 

 largest part of this pressure-resultant is due to the boundary-waves. 



The oscillations in the velocity of the boat; conclusions regarding 

 the wave-making resistance. 



Before closing the description of the qualitative results, one very interest- 

 ing effect of the dead-water must be mentioned, although it probably does not 

 occur so markedly on the sea, as it did in the experimental tank. In spite 

 of the boat-model being drawn through the water by a constant weight, I 

 never succeeded in giving to it a steady speed, when a layer of fresh 

 water covered the salt water in the tank. It was on the contrary, quickly 



