PADDLE WHEELS. 29 



PADDLE WHEELS. 



38. The arms, segments, and bolts of paddle wheels immersed below and in the neigh- 

 bourhood of the water line, in the Mediterranean and between the Tropics, are found 

 to decay very rapidly, much more so than I ever observed in the Channel or North 

 Sea. Having to remove several segments, arms, and bolts at Malta, in order to substitute 

 new in their places, after having been a long time in use, the oxidation exhibited in twenty-four 

 hours on those new ones which were turned unprotected into the water was astonishing : large 

 blotches of rust were distributed over the surfaces of the iron, which, when rubbed off, left 

 corresponding holes, eaten to a considerable depth. On strict examination, I was led to 

 attribute it mainly to the effect of the wash or spray dashing on the wheels when in harbour, a 

 ripple or ground swell being generally felt in the finest weather, causing them alternately to 

 dip into the water : the great heat of the sun speedily evaporating a portion of the water, 

 which I have often taken at 81 Fahrenheit, leaves either brine or crystals of salt adhering 

 to them, which of course becomes powerfully corrosive. 



We must look, however, to another agent as the principal cause of destruction, when under 

 weigh as well as in harbour. Some months after the above, I observed in the mole at 

 Gibraltar, where the water is so clear that the bottom of the ship can be distinctly seen from 

 alongside, that marine vegetation, barnacles, and lime were existing in great abundance in the 

 neighbourhood of the paddle wheels, while every other part of the bottom was comparatively 

 clean : this I pointed out to a friend, who immediately crossed with me to examine the 

 opposite side, where the same appearances were presented, no part of the wheel being nearer 

 than a foot or eighteen inches to the ship's side. 



Hence I concluded, that to the galvanic influence of the copper sheathing was this effect 

 owing, which became thus protected at the expense of the various parts of the paddle wheels : 

 this influence has before been suspected, allusion being made to it in the instructions for Her 

 Majesty's steam vessels ; but its effects, I believe, had never been demonstratively exhibited 

 hitherto. The circumstance was rendered still more remarkable, from the fact of the vessel 

 having just arrived from a voyage of eight days; the water in this particular place being 

 agitated more, and driven with a greater velocity, than on any other part of the bottom. 



It was remarked often in the Mediterranean, by persons in the paddle wheels, which were 

 the constant bathing-machines in harbour, that the ship's bottom must be very foul ; in which 

 opinion I was a party : to this was attributed any falling off in the speed of the vessel ; and a 

 " hog," or cleansing machine, was obtained to clear the same : but the bottom, when the 

 vessel was taken into dock at Woolwich, was found, as before noted, to be kept sufficiently 

 clean by constant employment at sea, no weeds or barnacles being found, except in the 

 vicinity of the wheels. 



Since the above observations, I have examined at Woolwich the wheels and bottoms of all 

 Her Majesty's steam vessels that have returned from the Mediterranean and West India 

 station ; the bottoms being clean, except the parts immediately abreast the wheels, which 

 were thickly covered with large barnacles, &c. ; and in one instance in particular, the teredo 

 worm had nearly bored his way through the ship's side in this spot only ; the copper sheathing 



