570 



NA TURE 



[April i i, 1895 



apparatus, construcied for the purpose, illustrated this fact by 

 showiti" that the horizontal oscillations at the deck and bottom 

 of the ship respectively are in opposite directions at the sanne 

 instant of time. At a certain height above the keel there is no 

 horizontal oscillation, this being therefore the locus of the axis 

 of torsion. Maxima and minima of the turning moment at each re- 

 volution depend on the number of cranks, the amplitude of the 

 oscillations beinj; mostly dependent on steam distribution. These 

 oscillations are periodic, and likewise have their nodular points. 

 The author next proceeds to treat the points mentioned 

 mathematically in the case of a prismatic rod. He shows that 

 the number of vibrations is proportional to the speed of pro- 

 gress of vibration. Substituting a ship's body, he finds that 

 this speed of progress remains constant for similar ships, and 

 also that the number of torsional vibrations varies in an indirect 

 proportion with the length of the ship. For a better under- 

 standing of these points, we must refer our readers to the original 

 paper and the diagrams by which it is illustrated. That engines 

 of special construction will cause no vibration if placed just 

 above the nodular point, is also true for torsional vibrations. 



Mr. Mallock, who, it may be slated, has done much excel- 

 lent work for the Admiralty in connection with this subject, 

 dealt in his paper with " the determination of the direction and 

 magnitude of the forces and couples which arise from the un- 

 balanced moving parts of marine engines." Something may be 

 done, the author said, towards balancing an engine by the 

 proper disposition of the pistons, connecting-rods, and cranks ; 

 but it does not seem practicable to produce a com- 

 plete balance in any ordinary engine without having recourse 

 to counterbalance weights. In order to determine the weights 

 required, the author has produced a geometrical construction 

 showing, by the aid of arithmetic only, the resultant force and 

 couple due to the unbalanced moving parts of any engine. With- 

 out the aid of the diagrams >hown at the meeting, it would be 

 impossible to make the explanation clear, even if space permitted 

 us to give the details in full. It will be sufficient to say that the 

 engine is divided up into its component parts, to each of which 

 a value is given, and in this way the resultant force is found and 

 the resultant couple determined. Having got all the information 

 necessary to assign the magnitude and direction of the force and 

 couple which will completely balance the engine, if the force 

 could be applied at the centre of gravity of the moving parts, it 

 would merely remain to decide what weights should be used to 

 produce the required effect. In general the construction of the 

 engine makes this inconvenient, if not impossible, and other 

 positions for the counterbalance wtights must be found. This 

 aspect of the problem is then considered in detail by aid of the 

 figures. 



The second part of the paper was devoted to showing how 

 the frequency of vibration of any ship, loaded in any manner, 

 can be found by models, and that all the data for shaping the.'-e 

 models can be readily obtained from curves which would be in 

 the hands of the ship designer. .Vn example of the apparatus 

 used was shown, the author giving a practical illustration of its 

 working. The course pursued is to make an exaci copy of the 

 ship on a very small scale, exactly proportional in all dimen- 

 sions and identical in material. It is known by theory that ih-i 

 frequency of vibration of the model and ship will be inversely 

 proportional to their lengths. The model is replaced by an 

 exact copy on the same scale, made of some other material — 

 wood— the frequency of the new model differing from that of 

 the former in the ratio 



7' 



where y„>, </,, and p, are the respective elasticities (Young's 

 Modulus) and densities of the wood and the material of the 

 ship. Next the wooden model is replaced by a jilank of the 

 same wood of uniform tbicknc-^s but variable breadth, the 

 breadth being such that the stiffncs of the plank against 

 bending at every cross section is proportional to the .siilTness of 

 the model at the coiresponding position. Wtights are fixed to 

 the plank in such a manner that the weight at any cross section 

 ia proportional to the weight at the corresponding section of 

 the model. Then the frequency of the plank, compared with 

 that of the model, can be ascertained by a formula. 



In the apparatus shown the plank was supj)urted by two 

 rollers tlung from two similar rollers, the lalicr resting on an 

 overhead railway. The plank was kept vibrating by a magnetic 

 apparatus, and a recording device was added. The rollers 



NO. I32S, VOL. 51] 



supporting the plank gave the position of nodes. It is only 

 when the rollers are at the positions where the nodes would be, 

 if the plank was free from all constraint, that the frequency of 

 the plank will be related to that of the ship as given by the 

 author's formula. The natural nodes are found by varying the 

 position of the rollers until the frequency is a maximum for the 

 type of vibrations under consideration. 



The method here introduced by Mr. Mallock is interesting 

 and ingenious, but how far it is applicable to the needs of the naval 

 architect, or whether the average ship builder, if he wish to 

 reduce vibration, will prefer the former method of adjusting the 

 engine to the known conditions of the ship after she is built, 

 are questions which experience alone can decide. 



The paper of Mr. Robinson and Captain Sankey dealt 

 largely with the question of vibration in connection with electric 

 light engines, the problem of vibration in the hull of a vessel 

 bring thus eliminated. Here again a number of diagrams were 

 used which we cannot now reproduce, and our abstract of this 

 paper must be therefore brief. Investigation showed that in the 

 case of an electric light station the high speed vertical 

 engines, each 200 indicated horse-power, with two cranks 

 set opposite each other, and run at 350 revolutions per minute, 

 were mounted on a large slab of concrete. The en- 

 gines being vertical, the moving parts had to travel through 

 a greater distance during the upper half of the revolution of 

 the crank pin, than during the lower half. Calculation 

 showed that each line of parts singly tended to lift the engine 

 at up-strokeby about 35 tons, audit tended to depress it 2'3 tons. 

 Therefore twice in a revolution a net lifting power of one ton 

 acted upon the engine, and changed an equal number of times 

 into a depressing power of about i '2 tons. The result was a 

 "pumping action" on the water-soaked soil beneath the con- 

 crete slab, and in this way vibration was conveyed to surround- 

 ing buildings. The action, it will be seen, was due to the 

 angular movement of the connecting rods, a feature which llerr 

 Schlick said might be neglected ; a point in which the authors, 

 naturally, and also Mr. .Mallock, l>yno means agreed with him. 



An arrangement of two engines with their framings rigidly 

 connected, and having three cylinders each, was proposed, the 

 object being to neutralise the endways rocking or tilting ten- 

 dency, and also to give freedom from tendency to vary the 

 downward pressure. 



.\ discussion followed the reading of these papers. 



Mr. Hiik's paper described a new way ol carrying out 1 

 known investigation. Whether the new way is better than the 

 old way, is a point which may be decided by experience. The 

 last paper of the meeting, that by .Mr. Martin, was of a dis- 

 appointing nature. Marine engineers have long been asking 

 for an explanation of the hitherto unexplained fact — if fact it 

 be — that " induced draught " is so much better for_ boilers than 

 "forced draught." Mr. Martin's experiments were quite 

 beside the mark. 



The summer meeting of the Institution will be held in Paris, 

 commencing on June 1 1. 



QUESTIONS BEARING ON SPECIFIC 

 STABILITY} 



AT the suggestion of your President, I beg to submit three 

 questions to the notice of this Society. They bear on a 

 theoretical problem of much importance, namely, the part played 

 in evolution by "organic stability." 



The questions arc especially addressed to those who have had 

 experience in breeding, but by no means to breeders only ; nor 

 are they addre>scd only 10 entomologisis, being equally ap- 

 propriate to the followers of every other branch of natural his- 

 lory. I should he gratefid for replies relating to any species of 

 animal or plant, whether based on personal observation nr re- 

 ferring to such observations of others as are still scattered 

 through the wide range of periodical literature, not having yet 

 found a place in standard works. The questions arc for in- 

 formation on : — 



(l) Instances of such strongly marked peculiarities, whether 

 in form, in colour, or in habii, as have occasionally appeared in 

 a single or in a few individuals .Tmorg a brood ; but no record 

 is wanted of monstrosities, or of such other characleristicj •» ■ 

 arc clearly inconsistent with health and vigour. 



' A paper read :ii ihe Eniomological Society, April 3, 189S, by l"r«nci» 

 Cilton, K.R.S. 



