68 EKMAN. ON DEAD-WATER. [NORW. POL. EXP. 



accelerated up to a higher velocity than its mean velocity, it then slowed 

 down, took a new leap, and so on: in this way the velocity performed oscill- 

 ations, which where sometimes very considerable. 



The diagrams PI. VII— VIII, showing the changes in the velocity with 

 the time, during some selected experiments, may illustrate this. The number 

 on the left of each diagram, refers to the number of the experiment, found in 

 the second column of the tables (in the next section of this chapter). The time 

 is reckoned horizontally from left to right, the distance between each two ad- 

 jacent vertical lines representing an interval of 1*09 seconds (in diagram No. 

 28S, it represents 0*436 seconds). The first part of the curve is left out on 

 every diagram, so that the epoch t — 0, at which the boat started, must be 

 imagined to be somewhere to the left of the figure. The velocity of the boat 

 is reckoned vertically, and the scale, in cm. per second is given to the left of 

 each figure: in all diagrams except No. 288. two intervals are equal to 1 cm. 

 per second. The velocity is, at equal intervals of time, marked by small circles, 

 and through them the curves are drawn. 



The diagrams 18—47 refer to experiments in homogenous water. They 

 show a velocity, gradually increasing, if the initial velocity given to the boat 

 was too slow, or decreasing if it was too fast. Only in the experiments in 

 which no steering string was used (30—35, 41—47) did it sometimes happen, 

 that the velocity increased and again slackened, on account of the boat mak- 

 ing sheerings from its straight course. 



When there was a layer of fresh-water on the top of the salt-water, the 

 motion of the boat took place quite differently. The changes of velocity in 

 such a case, are shown typically by the diagrams 77—83 (depth of the fresh- 

 water layer 5 cm.: spec, gravity of the salt-water 1*032). Experiment 77 was 

 made with a towing-force of 0'8 gr. (about one third of the maximum resi- 

 stance): in the course of about 40 seconds, the velocity goes through 3 com- 

 plete and quite distinct oscillations, but the amplitude of the variations is 

 not very great |0'4 cm. per second). The oscillations become more and 

 more marked, as the towing force is increased, and simultaneously, they take 

 a longer time. In experiment 80 (towing force 2"0 gr.) the velocity dropped 

 from a maximum of 9 cm. per second to a minimum of 5 cm. per second, 

 and one oscillation seems to have required about 25 seconds. When the towing- 

 force was 2 - 2 gr. (diagram 82), which was very near to the maximum resi- 



