56 EKMAN. ON DEAD-WATER. [norw. pol. exp. 



The wheel w 2 alone, weighs 1-68 gr. To get towing forces less than 

 half this (0-84 gr.), counter-weights c were used in the manner shown by the 

 figure (actually, the cocoon thread of the counter-weights was always un- 

 hooked, when — as in the figure — the boat was being towed by more than 

 the aluminium-wheel's own weight). The weights and counter-weights were 

 made so as to produce towing forces of even decigrammes, attention being 

 paid to the friction in the wheels. 



Hie registration of the velocities of the boat-model was effected on a 

 telegraphic tape, which received two sets of marks side by side — one set of 

 "time-pricks" at equal intervals of time, and one set of "way-pricks" at equal 

 intervals of distance covered by the boat-model. The apparatus employed for 

 this purpose is represented by Fig. 5, PI. I. The current from an electrical 

 battery o of 4 volt passes through anyone of the two induction-coils i x and i 2 , 

 when the concerning circuit is closed. The circuit (1) is closed each 0.435 of 

 a second, through the point of a pendulum p x touching a mercury meniscus m x . 

 The circuit (2) passes through the spindle of the registering-wheel (w 1( Fig. 4) 

 and is closed when a steel point p 2 on the wheel, touches the mercury menis- 

 cus m 2 . Except when the boat-model moved at very high speeds, two such 

 points were used, placed on the wheel, diametrically opposite to oneanother. As 

 the circumference of the wheel was 10 cm., the circuit was then closed once 

 for each fifth cm. covered by the boat-model. The telegraphic tape t moves 

 between the secondary poles s,, s 2 , s s of the induction-coils (the cylindrical pole 

 s 3 is common to both the coils) and is pierced by the sparks. The spark-length 

 is considerably diminished, and the effect of the sparks increased, by a con- 

 denser c of three metal discs, connected with the poles s 1? s 2 , s 3 . If the 

 meniscus m is not too extensive, the interruptor of the induction-coil has time 

 enough to break the circuit but not enough to close it again, before the point 

 p has left the mercury. With this arrangement the current is never broken 

 at the meniscus, which remains clean and in working order very long. By 

 friction against a short piece of India Rubber tubing, the vibrations of the 

 hammer-interruptor was each time stifled, before the primary circuit was closed 

 the next time. The pieces of tape belonging to the separate experiments, were 

 registered and were read off at convenience. The punctures were easily seen 

 when the tape was held before an illuminated mirror. The "time-pricks" — 

 or usually each fifth one — were read off on the scale formed by the "way- 



