1904 



on the Motion of Viscous Substances, 



487 



A good idea of the lines on which this is done may be obtained 

 in this way. Imagine a cube of the viscous material standing on a 



Fig. 1. 



table (Fig. 1) to be gradually twisted over to the right by a force 

 applied along the top. Then the slower the block twists the more 

 viscous it is said to be. 



Tube Method. 



The viscosity of the substances at the more liquid end of the 

 benzine-pitch series, can be determined by observing the rate at which 

 they can be forced to move through tubes under recognised conditions. 

 I'ig. 2 shows how a layer in such a tube lying between two cross 





Untwisted 



Twisted 



". I, 6 

 Fig. 2. 



Fig. 3. 



sections at any moment, becomes bent forward at its centre as the flow 

 proceeds. The little square shown in the first position has taken up 

 a skewed shape in the second. This helps us to see that we have in 



