1888.J On the Heating Effects of Electric Currents. 109 



supply and drainage pipes. In canals the trapezoidal section is that 

 which experience has almost universally established as the best wher- 

 ever canals are carried through ordinary earth, and the rectangular 

 section is only adopted when the sides are composed of coherent 

 matter such as rock or masonry. The semicircular section for an I 

 open channel would not approximate to the shapes usually adopted in 

 canals, but it may be worth remarking that the outline of the 

 catenary of greatest area approaches more nearly to such shapes. 



IV. "On the Heating Effects of Electric Currents. No. III." 

 By W. H. Preece, F.R.S. Received March 15, 1888. 



I have taken a great deal of pains to verify the dimensions of the 

 currents, as detailed in my paper read on December 22, 1887, required 

 to fuse different wires of such thicknesses that the law 



C = a^ 3 / 2 



is strictly followed ; and I submit the following as the final values of 

 the constant " a " for the different metals : — 





Inches. 



Centimetres. 



Millimetres. 





10,244 



2,530 



80-0 





7,585 



1,873 



59-2 



Platinum 



5,172 



1,277 



40-4 





5,230 



1,292 



40-8 



Platinoid 



4,750 



1,173 



371 





3,148 



777-4 



24-6 



Tin 



1,642 



405-5 



12-8 



Alloy (lead and tin 2 to 1) 



1,318 



325-5 



10-3 





1,379 



340-6 



10-8 



With these constants I have calculated the two following tables, 

 which I hope will be found of some use and value : — 



