July io, 1891.J 



SCIENCE. 



23 



of the other. The greatest force would be a little way out, and 

 that, according to Faraday's observations, systematized and ex- 

 jiressed in the form of mathematical law by Sir William Thomson, 

 was where the ball would go. 



The next discovery of Faraday to which the lecturer called at- 

 tention was one of immense signi6cance from a scientific point of 

 view, the consequences of which were not even vet fully under- 

 stood or developed. He referred to the magnetization of a ray of 

 light, or what was called in more usual parlance the rotation of 

 the plane of polarization under the action of magnetic force. It 

 would be hopeless to attempt to explain all the preliminaries of the 

 experiment to those who had not given some attention to those 

 subjects before, and he could only attempt it in general terms. It 

 would be known to most of them that the vibrations which con- 

 stituted light were executed in a direction perpendicular to that 

 of the ray of light. By experiment he showed that the polari- 

 zation which was suitable to pass the iirst obstacle was not suitable 

 to pass the second, but if by means of any mechanism they were 

 able, after the light had passed the first obstacle, to turn round the 

 vibration, they would then give it an opportunity of passing the 

 second obstacle. That was what was involved in Faraday's dis- 

 covery. As he had said, the full significance of the experiment 

 was not yet realized. A large step towards realizing it, however, 

 was contained in the observation of Sir William Thomson, that 

 the rotation of the plane of polarization proved that something in 

 the nature of rotation must be going on within the medium when 

 subjected to the magnetizing force, but the precipe nature of the 

 rotation was a matter for further speculation, and perhaps might 

 not be known for some time to come. 



When first considering what to bring before them, tlie speaker 

 thought, perhaps, he might include some of Faraday's acoustical 

 experiments, which were of great interest, though they did not 

 attract so much attention as his fundamental electrical discoveries. 

 He would only allude to one point which, as far as he knew, had 

 never been noticed, but which Faraday recorded in liis acoustical 

 papers. "If during a strong steady wind, a smooth flat sandy 

 shore, with enough water on it, either from the receding tide or 

 from the shingle above, to cover it thoroughly, but not to form 

 waves, be observed in a jjlace where the wind is not broken by 

 pits or stones, stationary undulations will be seen over the whole 

 of the wet surface. . . . These are not waves of the ordinary 

 kind ; they are (and this is the remarkable point) accurately 

 parallel to the course of the wind." When he first read that 

 statement, many years ago, he was a little doubtful as to whether 

 to accept the apparent meaning of Faraday's words. He knew of 

 no suggestion of an explanation of the possibility of waves of that 

 kind being generated under the action of the wind, and it was, 

 therefore, with some curiosity that two or three years ago, at a 

 French watering-place, he went out at low tide, on a suitable day 

 when there was a good breeze blowing, to see if he could observe 

 anything of the waves described by Faraday. For some time he 

 failed absolutely to observe the phenomenon, but after a whUe he 

 was perfectly well able to recognize it. He mentioned that as an 

 example of Faraday's extraordinary powers of observation, and 

 even now he doubted whether anybody but himself and Faraday 

 had ever seen that phenomenon. 



Many matters of minor theoretic interest were dealt with by 

 Faraday, and reprinted by him in his collected works. The 

 speaker was reminded of one the other day by a lamentable acci- 

 dent which occurred owing to the breaking of a paraffin lamp. 

 Faraday called attention to the fact, though he did not suppose he 

 was the first to notice it, that, by a preliminary preparation of the 

 lungs by a number of deep inspirations and expirations, it was 

 possible so to aerate the blood as to allow of holding the breath for 

 a much longer period than without such a preparation would be 

 possible. He remembered some years ago trying the experiment, 

 and running up from the drawing-room to the nursery of a large 

 house without drawing any breath. That was obviously of great 

 practical importance, as Faraday pointed out, in the case of dan- 

 ger from suffocation by fire, and he thought that possibly the ac- 

 cident to which he alluded might have been spared had the 

 knowledge of the fact to which Faraday drew attention been 

 more generally diffused. 



The question had often been discussed as to what would have 

 been the effect upon Faraday's career of discovery had he been 

 subjected in early life to mathematical training. The first thing 

 that occurred to him about that, after reading Faraday's works, 

 was that one would not wish him to be anything dii!ferent from 

 what he was. If the question must be discussed, he supposed they 

 would have to admit that he would have been saved much wasted 

 labor, and would have been better en rapxjort with his scientific 

 contemporaries if he had had elementary mathematical instruc- 

 tion. But mathematical training and mathematical capacity were 

 two different things, and it did not at all follow that Faraday had 

 not a mathematical mind. Indeed, some of the highest authori- 

 ties (and there could be no higher authority on the subject than 

 Maxwell) had held that his mind was essentially mathematical in 

 its qualities, although they must admit it was not developed in a 

 mathematical direction. With these words of Maxwell he would 

 conclude: " The way in which Faraday made use of his idea of 

 lines of force in co-ordinating the phenomena of electric induction 

 shows him to have been a mathematician of high order, and one 

 from whom the mathematicians of the future may derive valuable 

 and fertile methods." 



THE "SUBMARINE SENTRY." 



At a recent meeting of the Royal United Service Institution, 

 London, a lecture upon sounding machines was given by Profes- 

 sor Lambert of the Royal Naval College, Greenwich. In the 

 course of the lecture (some details of which appear in Engineering 

 of June 36) a description was given of an instrument called a 

 "submarine sentry," which has been successfully experimented 

 with on some ships of the British navy. It is the invention of 

 Mr. Samuel James, a civil engineer. 



As described by the lecturer, the sentry is intended to give a 

 continuous under- water look-out, and to automatically give warn- 

 ing of the approach of shallow water. It consists of an inverted 

 wooden kite, which can be trailed at the stern of a vessel at any 

 required depth to forty-five fathoms. On striking bottom, the 

 blow, acting on a projecting trigger, releases the slings of the kite 

 and causes it to rise to the surface and trail in the wake of the 

 vessel. At the instant of striking, the sudden loss of tension in 

 the wire sounds a gong attached to a winch on board the ship. 

 The wire used is of steel, and of the highest tenacity attainable. 

 Its diameter is 0.067 of an inch, and it is capable of bearing a 

 stress of fully a thousand pounds. During towing the vibration 

 of the wire causes a continuous rattle in a sounding box, and the 

 cessation of this noise gives an additional indication when the 

 sentry has struck the bottom. The vertical depth of the kite at 

 any time is indicated on the dial plate of the winch. The curve 

 formed by the wire while towing is concave downwards, and at 

 first sight it would appear as if this curve would change its form, 

 and the sinker trail further astern and at less depth when the 

 ship's speed was increased. Professor Lambert had carefully 

 plotted out this curve, and showed the results on a diagram. By 

 a mathematical analysis he showed that the instrument would 

 remain constant in its record at any speed of the ship between 

 five and fifteen knots. The weight of the kite is equal to, and is 

 therefore neutralized by, its own buoyancy, and the weight of the 

 wire is negligible compared to the forces due to the motion 

 through the water. 



The forces which i-emain to be considered are, (1) the fluid 

 pressure on the kite, (3) the fluid pressure on the under side of the 

 wire, and (3) the tension of the wire. The latter is the result of 

 the two former. Pressures due to fluid motion vary nearly as the 

 square of the velocity. If, therefore, the velocity of the ship be 

 doubled, forces (1) and (2) will each be multiplied by four, the 

 three forces will all be changed proportionally, and there wiU be 

 no change in the direction in wiiich they act. This is only put 

 forward as a rough explanation of the phenomena, but that it is 

 practically true has been, it is claimed, corroborated by practical 

 tests, — the depth of the sinker not varying more than half a 

 fathom in thirty at speeds of from five to thirteen knots, above 

 which speed the instrument is not designed to be used. 



There are two descriptions of kite, one set at an angle to give 



