On Zine Roofing. 315 
The power may be increased almost infinitely, by increasing the 
size of the temporary magnets, by increasing the number and the 
length of the bars of the permanent magnets, by increasing the width 
and length of the metallic ribbons surrounding the temporary mag- 
nets, and with these by increasing the power of the galvanic battery. 
Arr. XII.—On Zine Roofing; by Prof. L. D. Gaus, of the New 
ork University. 
Read before the Mechanics’ Institute, June 6, 1837. 
: TO THE EDITOR. 
Sir—The following remarks are intended as a reply to a paper by 
my friend Prof. Caswell, of Brown University, and which appeared 
in the 31st volume of this Journal. In March, 1836, I published in 
the Mechanics’ Magazine of this city, (New York,) some remarks on 
zinc as a roofing material, substituted for slate, copper, tin or tiles, 
in which I stated the bad results that had followed the experiments 
made on zinc in this city, @nd pointed out the defects of that metal. 
Parts of that paper were copied into the newspapers of the day, from 
which I suppose Prof. C. obtained his idea of its correctness, inas- 
much as he has both misconceived and misquoted my paper. In 
the present remarks I shall first state the positions taken in my for- 
