300 Mr. Horsf all on Copper Sheathing. [April, 



obligations to Sir Byam Martin, the Comptroller, and Sir Robert 

 Seppings, the Surveyor of the Navy, ibr the interest they have 

 taken, and the zeal they have shown in promoting these 

 researches ; and without stating how much I owe to the care, 

 attention, and accuracy of Mr. IS^olloth, Master Ship-wright, and 

 Mr. Goodrich, Mechanist in the Dock-yard at Portsmouth, in 

 superintending the execution of many of the experiments. 



Article VIII. 



State of the Copper on a Ship's Bottom with Excess of Protection, 

 and of the Cast Iron Protector^ after a Voyage to the West 

 Indies. 



We have received the following communication from Dr. 

 Traill, of Liverpool, who states, that Mr. Horsfall is the owner 

 of the vessel mentioned in his letter, and adopted Sir Humphry 

 Davy's method of defending the copper sheathing of ships' 

 bottoms, immediately on its promulgation. Dr. Traill adds that 

 the surface of the protecting metal to that of the copper some- 

 what exceeded the proportion assigned by Sir H. Davy, being 

 from Y^-o to -^-^ of the whole copper surface of the ship. 



The first fact mentioned in Mr. Horsfall's letter has been long 

 known. Mr. Daniell formed a similar substance artificially, 

 eight yeais since, by immersing a cube of grey cast iron in 

 diluted muriatic acid (see Journal of Science, vol. ii.) ; and in 

 the same Journal (vol, xii. p. 407), mention is made of a cast 

 iron gun, which by long immersion in sea water was incrusted 

 to the depth of an inch with a substance having all the exterior 

 characters of impure plumbago. As to the state of the Tickler's 

 bottom, it is obviously owing to the copper having been over 

 protected. (See Annals of Philosophy, vol. viii. N. S. p. 364.) 

 It is stated in the place referred to, that the requisite proportion 

 of defending surface to that of the copper, as far as had then 

 been ascertained, is somewhere between -^i^ and -^^-^ ; the pro- 

 portion at present adopted in the Royal Navy, we believe, does 

 not exceed -j^. In the preceding paper from the Philosophical 

 Transactions, our readers will observe that on sheets of copper 

 defended by quantities of cast iron and zinc in a less proportion 

 than YTo- "0 deposition of alkaline matter or adherence of weeds 

 ook place, and the surface, though it had undergone a slight 

 degree of solution, was perfectly clean, ** a circumstance of 

 great importance, as it points out the limits of protection ; and 

 makes the application of a very small quantity of the oxidizable 

 metal more advantageous in fact than that of a larger one." 



Mr. Horsfall's letter is a candid statement of a fair experi- 

 ment, and the result, as far as it can be learnt, is the saving of 



