PROSPECTS OF STEAM-NAVIGATION. 



the water everywhere at right angles to that course, and no backward ci.rrent 

 would be produced ; but by giving them a position between these two extn 

 that is to say, inclined at some oblique angle to the course of the vessel 

 the revolution of the wheel will cause them to exert a certain portion of their 

 force on the water in producing a backward current : and that particular obli- 

 quity should be given to them which will make that backward current most 

 effective. 



The calculation of this obliquity requires the application of the prim-: 

 of mathematical science, and admits of a clear and definite solution, it is 

 found, however, that the most effective obliquity for the propelling surface is 

 not the same for all distances from the centre of the wheel, and consequently 

 if the best possible form be given to the propelling blades, they must be shaped 

 according to a certain spiral to be determined by conditions depending upon a 

 variety of circumstances connected with the propeller and the vessel itself. 



Some projectors, ignorant of these scientific principles, have .constructed 

 these propellers with plane surfaces, without the spiral form. Such is the 

 patented contrivance called Loper's propeller. They are consequently and 

 most obviously inefficient. 



But besides the proper adjustment of the obliquity of the propelling surfaces, 

 the experience of Captain Ericsson soon proved that the parts of the blades 

 near the centre of the wheel were not only inefficient for propulsion, but formed 

 an impediment to the progress of the vessel. It was for this reason, among 

 others, that he cut away those parts of the blades near the centre, retaining 

 only the more remote portions, and supported these by bolting them on to the 

 hoop already described. 



Such being the general character of this propelling instrument, it will be ap- 

 parent that in every position which it can assume in the water, it must pro- 

 duce nearly the same propelling effect. However the ship may pitch or roll, 

 or however unequal the surface of the sea may be, it will always produce the 

 backward current, without any great variation of effect. 



The circumstances which prevent the co-operation of the power of steam 

 with that of the sails in the steam-vessels now in use, will not operate with a 

 propeller of this form, inasmuch as its efficacy will be altogether independent 

 of the careening of the ship ; but although this defect is removed, the sub- 

 merged stern-propellers are still subject to objections from which even the 

 common paddle-wheels are free. Being permanently submerged and 'liable to 

 accidental fracture and derangement from various causes, they are inacces- 

 sible, and cannot be repaired at sea ; but besides this, when the object in view 

 is to take full advantage of the power of the sails, that of the machinery being 

 suspended, the submerged propeller becomes an obstruction, more or less con- 

 siderable, to the progress of the vessel. 



An invention, however, recently patented by Captain Ericsson, has finally 

 removed this difficulty, and placed it in the power of the commander at any time 

 within the space of five minutes to raise the propeller out of the water, or to 

 submerge it, so as to convert for all intents and purposes a steamer into a sail- 

 ing-vessel, or a sailing-vessel into a steamer, as he may see fit. 



The shaft on which the propelling-wheel is fixed is provided with a simple 

 mechanism within the vessel by which it may be easily at any time drawn out 

 of the nave of the wheel. Tne wheel itself is sustained by a powerful vertical arm, 

 the upper end of which is attached to a strong axis, which enters the vessel 

 parallel to the main axis of the wheel and above the summit of the wheel. To 

 this axis within the ve ssel is attached a piece of mechanism by which it may 

 be turned through hah a revolution by the power of two men with such force 

 that the propeller will be made to perform half a revolution round the upper 



