378 Proceedings of the Royal Irish Academy. 
angles of inclination of the six screws of a coreciprocal system," and 
then I mentioned the first of the three just written as an example. At 
the time I did not know, nor indeed did I know nntil a few months 
ago, that of these six f ormnlge three were merely identities. The other 
three which do tell us something ahout the coreciprocal system arc 
given above. I may take this opportunity of saying that after some 
years of labour on the part of Herr Harry Graveling, the German 
Edition of the Theory of Screws " has at last appeared. In a volume 
of 619 pages he has translated and welded into a continuous whole, 
not only the original volume on the Theory of Screws " published in 
1876, but also the principal papers on the subject which I have since 
laid before the Academy. The title of the book is " Theoretische Me- 
chanik Starrer Systeme, von Sir Hob. S. Ball," Herausgegeben, von 
Harry Gravelius : Berlin, Georg Ecimer, 1889. 
