=Submarines vs. Submarines.=--Submarines to search for and sink other submarines have been proposed in all sorts of forms and advocated in the press under various titles, such as the "Bloodhounds of the Sea" and other fantastic and sensational captions. Submarines cannot fight submarines, because they cannot see each other, and if they are fitted with noiseless machinery they cannot hear each other. Therefore one might put thousands of submarines in the great ocean, and so long as they kept submerged the chance of their ever finding or colliding with one another would probably be not once in a year. Derelicts have been known to keep afloat on the ocean for years, although constantly searched for as a menace to navigation. Here the searchers have had sight to aid them, and the object of their search has floated on one plane, the surface of the water, while submarines may navigate or remain at rest at various planes up to a depth of about two hundred feet, which is equivalent to multiplying the area of the ocean to be searched several times, and that in darkness, without the aid of sight to assist. It is ridiculous to think that anything can be accomplished except by the merest chance by one submarine searching for another. Our attention will now turn to consideration of devices of the second class; namely, those which have been offered as a means of defence against the submarine. =Blinding Searchlight; Blinding Apparatus.=--_Blinding devices_ have been proposed which aim to direct powerful searchlights against the periscope so as to blind the commander. These are schemes based on very false notions. Submarine commanders frequently have to con their ship against the sun's rays, and have colored glasses to enable them to withstand the intensity of the sun's rays, so that it would be impossible to blind them this way. Further, I cannot imagine a more desirable target for a commander to direct his torpedoes against than a bright spot, either on the surface or submerged, as he knows the searchlight is probably on what he wants to hit; it becomes an illuminated bull's-eye for his target. Again, it has been proposed to blind the periscope by putting a film of oil on the surface to obscure the object glass of the periscope when it emerges through this oil, and a member of one of the British commissions told me he knew of shiploads of oil being pumped overboard, possibly for this purpose, or to show the course of a periscope through its "slick." Some periscopes have been built with means for squirting alcohol, gasoline, or other substances to clear the object glass if ice or salt forms on it. A device of this kind would clear off the oil.