116 Mr. Snow Harris on Lightning Conductors 
fessor’s observation of the effect of nitrate of mercury in 
preserving the inactive state ; but it was also necessary to 
protect the wires at and above the surface of the acid from 
the corroding action of its fumes by a coating of wax or glass. 
By these means the inactive condition of the iron was main- 
tained ; but another obstacle then arose in the crystallization 
of nitrate of mercury, by which the cells after twenty or thirty 
hours’ use were generally broken ; and this could not be sur- 
mounted but at the sacrifice of some power, by diluting the 
acid, which the presence of the nitrate permitted, and the 
substitution for the cells of pipe-clay of others made of wood. 
I cannot give comparative results obtained with iron and 
platina batteries, but I may mention that with an arrange- 
ment of six cylinders of sheet iron, each containing thirty-six 
square inches in strong nitric acid, without mercury, and as- 
sociated with zinc plates of half their size, a current was evolved 
which for several hours ignited charcoal, or the whole of a 
strip of platina six inches in length by one-eighth of an inch in 
width. The construction of this battery was imperfect in se- 
veral respects, particularly in the porous vessels being much 
too thick ; there is, therefore, reason to suppose that an equal 
power might have been obtained from a less number of cells. 
As some progress in advance of the Professor’s experiment 
in which the current ceased with the solution of the film of 
peroxide of lead, this communication may possibly be deemed 
worthy of being recorded : but the importance of my results 
in relation to the proposed object of the experiments is, I 
think, materially affected by the discovery of Mr. Cooper, as 
given in your last Number, of the application of charcoal and 
other forms of carbon Jis a substitute for platina in the voltaic 
arrangement of Mr. Grove. 
I have the honour to be. Gentlemen, &c. 
5^, King’s Road, Brighton, ThoMAS HawKINS. 
Jan. 10th, 1840. 
XX. 071 Lightning Conductors^ and the Lffects of Lightning 
on Her Majestfs Ship Rodney and certain other Ships of 
the British Navy : being a further examination of Mr. 
Sturgeon’s Memoir 07i Marine Lightning Conductors, By 
W. Snow Harris, Esq,^ F.R.S., ^c. 
[Illustrated by Plate 1,] 
To the Editors of the Philosophical Magazme and Journal. 
Gentlemen, 
1. TN my former communication (L. and E. Phil. Mag. vol. 
xiv. p. 461.) I considered the nature of a well-known 
