NO. 16.] APPARATUS AND METHODS. 55 



the 4 points of support s, s, s, s' are so chosen that the greatest deflection 

 at any point of the beam should be the least possible. Two horizontal iron 

 screw bolts (b, b, Figs. 1 and 2) join the bottom beams together and receive 

 the pressure against the sides; similarly an iron cramp c holds the upper 

 beams together. By these arrangements it was calculated, that the deflection 

 of any part of the beams to which the panes are cemented, does not exceed 

 O'l mm. The wooden springs (ss) also protect the tank from too great twist- 

 ing forces; if the two tables which support the tank be not exactly in one 

 plane, two of the springs would bend a little more than the two other one's 

 and would sustain only a few kilogrammes more pressure. 



The panes are so put in that, when broken, one may easily be taken 

 away and replaced by a new, without disturbing the others. They were laid, 

 one at a time, in spacious channels of tin-plate 1 (t, Figs. 1 and 3) filled with 

 "marine glue" (m, Fig. 3). The glass having been left for some hours to sink 

 down into the pitch, the wooden laths I were laid directly on the glass and 

 screwed to the beams. The elastic "marine-glue" allows the panes to expand 

 freely by heat. Other details and dimensions are shown by the figures. 



The wooden frame was made by Mr. E. G. Ekstrand in Stockholm, and 

 is of excellent workmanship. The tank has now been in use for 3V2 years, 

 and as far as can be seen, the frame has not warped the very least. 



The towing-apparatus, attached to one end of the tank, is arranged for 

 towing with constant force, not at constant speed. It is shown in Fig. 4, PI. I. 

 The towing-thread — a cocoon thread weighing about 3 mgr. per meter — is 

 laid over the wheel w x , which also serves to record the distance covered by 

 the boat-model and therefore has a circumference of exactly 10 cm. Further, 

 the thread is brought under the aluminium wheel w 2 , on which towing- weights 

 t are hung, and its end is fixed to the hook h. Thus, the boat-model is 

 dragged twice the distance descended by the wheel w 2 . 



An additional "starting weight" s can be hung directly on the axis of 

 w s , to bring the boat-model more quickly to its uniform speed. This starting 

 weight is lifted off on reaching the platform p, which is movable by soft 

 friction along the rail r and can be placed at different heights according as a 

 greater or lower initial speed is desired. 



Copper channels, being much more durable, would have been far preferable. 



