﻿On the Trisection of an Angle. 



861 



if a is small. For larger angles, numerical calculation is 

 the only process available. The accompanying table gives 

 the parts into which the angles entered in the first column 

 are divided by the construction ; the results are given in 

 degrees and decimals, of which the last fig are is only 

 approximate. 



o 

 30 



60 



90 



120 



lS-05 

 2033 

 30 95 

 41-68 



52-43 



9-90 

 19-34 

 28-10 

 3664 

 4514 



1005 

 20-33 

 30 95 

 41-68 

 52-43 



150 



It will be seen that the error appears to increase with the 

 gle, being about a degree for an angle of 90°, and nearly 



2^ degrees for an ano-le of 150°. 



Yours faithfully, 



T. J. Fa. Bromavich. 



LXXXV. Note on tlie Trisection of an Angle. 

 By J. R. Cotter, M.A* 



WITH reference to the point raised by Mr. Maskelyne 

 in the last issue of the Philosophical Magazine, viz., 

 the mechanical trisection of an angle, the following device 

 is perhaps the simplest that can be suggested. 



g AB, BC, CD are three 



, equal rods jointed at B and 



' C. If A lies on CD or on 



CD produced (as in the 

 figure), and if the angle 

 BDC is a, then the other 

 angles have the values given 

 in the figure, since ABC and 

 BCD are isosceles triangles. 

 If then we make the point 

 A slide on CD produced, till 

 the exterior angle DBE has 

 will give the required third 



a given value, the angle BDC 

 part. 



This method does not of course claim to be a " rule and 

 compass ' ; method. 



Communicated bv the Author. 



