348 



NATURE 



yAiigust 23, 1877 



necessity for some greater control over fast passenger trains was 

 thus rendered obvious. Througli the want of a larger amount 

 of brake power much time was lost on a journey, when the 

 stoppages were frequent, the drivers being compelled to slacken 

 speed at long distances (rem the stopping-places. It seemed, 

 indeed, scircely to admit of question that a system which 

 was deemed necessary in special cases might be advantageously 

 applicible in all cases ; that to render the control of a train 

 complete, brakes should be applied to all, or nearly all, 

 the wheels ; and that, at least, the driver, if not the 

 guards, should possess the power of promptly bringing 

 the whole into action. The truth of the principle was 

 row very generally admitted by the leading companies, 

 some of whom had already adopted continuous brakes, while 

 others were preparing to do so. Rather startling disparities 

 were disclosed during the experiments. Some of the disparities 

 were attributable to the contrivances being of comparatively 

 recent origin, but others were clearly owing to the principle upon 

 which the action of the brake was founded. As between the 

 air-pressure and the vacuum brakes there was a loss of i>\ 

 seconds, which in a train running sixty miles an hour was 

 equivalent to 180 yards additional space traversed in the stop. 

 Three of the experiments involved the application of all available 

 power for stopping. Sand was used, and was found to add 

 sensibly to the stopping power. On an average it made an 

 addition of i '30 per cent, to the retarding force otherwise brought 

 into play. The trials proved in a very striking manner the 

 great advantage of continuous brakes, for even in their least 

 effective form they afforded more than double the stopping 

 power of the usual hand brakes, whilst in their most effective 

 form the power was quadrupled. He was of opinion that no 

 system could be considered salisfactory which did not produce a 

 retarding power of at least S to 10 per cent, of the entire weight 

 cif the train, in other words, a power by which fast trains could 

 be stopped m from one-third to one-fourth less time than at 

 f resent. Obviously the stopping distance was primarily in- 

 fluenced by two considerations : — {\) The length of the interval 

 which elapsed between the brake being put in'o operation and 

 its taking an effective grip on the wheels ; and (2) the amount 

 of pressure brought to bear on each wheel, and the constancy or 

 otherwise of the action after the blocks had gripped the wheels. 

 The unpleasant sensation often experienced during quick 

 stoppages was produced by intermittent and fitful action. After 

 the brakes had been made to bite the wheels their hold 

 became relaxed, a slip took place, followed by successive 

 bites and slips, the latter giving rise to sudden accelerations 

 of speed. The action of a perfect brake should exactly resemble 

 that which gravity would cause if an ascending incline of 

 uniform gradient could be suddenly placed in front of the train 

 to prevent its motion. Under such conditions no inconvenience 

 or danger need be apprehended from the stoppage being accom- 

 plished within even a shorter distance than any that was effected 

 during the experiments. A valuable addition] of power, under 

 tie immediate control of the driver, would be afforded by the 

 fitting of brakes to the engine, and he was glad to find that the 

 recommendation of the Royal Commissioners in this respect had 

 met with prompt attention at the hands of the railw<ay compinies. 

 The question of the best material for brake blocks had of late 

 received a good deal of consideration, and it would seem that 

 cast-iron, and even steel, was fast superseding wood. It generally 

 happened that wheels did not become skidded until the speed of 

 the train had been materially reduced. It seemed desirable, 

 therefore, that for ordinary stjps the brake pressure should be 

 applied so as to act just short of skidding the wheels, the full 

 skidding power being only used in cases of imminent danger. 

 The general adoption of an effective system of cont.nuous brakes 

 on carriages which had to run from one line to another would be 

 productive of much advantage, for then, in breaking-up and re- 

 making a train at any junction station, the carriages would be 

 found ready-fitted with the requisite appliances for working. If 

 alhed companies could only agree to adopt the same system, 

 brake improvements would proceed with far greater rapidity than 

 at present, and public convenience would thereby be promoted. 

 The time had arrived not only when each system should be 

 scrutinised and tested in the most comp'ete manner, but when the 

 compafjies should clearly set before themselves the conditions 

 which a good continuous brake ought to supply. A study of 

 the diflerent methods which came under his (the lecturer's) 

 notice during the experiments pointed to the following 

 considerations as necessary in view of the provision of 

 perfect brake power fur heavy fast trains: — I. The brake 



power should be applied to all the wheels of all the 

 vehicles throughout the train. 2. The power by which the 

 blocks were forced upon the wheels should be adequate for skid, 

 ding the wheels on the speed becoming moderately reduced. 



3. The driver should have the whole of the brake power com- 

 pletely under his command, and be able to apply it at a moment's 

 notice, as he was the first person likely to discover any obstruc- 

 tion ahead, and was primarily responsible for the regard of the 

 danger signals. He could thus stop the train at once, and no 

 time would ba lost by his havin.; to signal danger to the guard. 



4. The guards should individually possess the like means of 

 applying the continuous brake, so that they might ba able to 

 stop the train without reference to the driver, on an emergency 

 which might manifest itself to them but not to him, such, for 

 instance, as a broken axle, or a carriage getting olT the line. 



5. The power in hand should be so susceptible of modification 

 that the driver should be able to apply a moderate amount only 

 for effecting ordinary stops, while he kept in reserve a proper 

 excess to be used only on emergencies, or on slippery rails. 



6. Full brake application should not require more than a very 

 moderate effort on the part of either driver or guar J. 7. Ttie 

 pressure should be steady, and distributed as equally as possible 

 over all the wheels, and, with the intervention of some elastic 

 medium, should act upon the wheels in such a way as to prevent 

 too sudden stopping or the snapping of chains, which produce 1 

 discomfort and inconvenience to the public. 8. The machinery 

 should be of simple constiuction, not likely soon to get out of order, 

 and admitting of being easily repaired. 9. Indication should be 

 constantly afforded to driver and guards that the brakes were in 

 proper condition to work or otherwise. 10. The power of 

 working the tender brakes and the van brakes by hand might be 

 advantageously retained. II. The brakes should be self-acting 

 in case of the severance of the train. 12. Automatic action 

 being provided, means should be furnished the brake attendants 

 for modifying that action instantaneously, according to the cir- 

 cumstances in which the train might be placed after an accident. 

 13. It would be dangerous, and therefore unadvisable, to give to 

 passengers any power over the brakes. Such seemed to be the 

 principal conditions necessary for realising the conception of a 

 perfect brake — a brake which would constitute an invaluable 

 instrument in contingencies of almost daily occurrence at some 

 place or another in the great railway netwoik of the country. 



REMARKABLE PLANTS 

 III.— The Sensitive Plant {Mimosa pudica). 



IN our ordinary popular conception of the difference 

 between the two kingdoms into which the organic 

 world is divided, we are apt to attribute to one a power 

 of spontaneous motion dependent on the possession of a 

 certain internal mental faculty to which we apply the 

 term voluntary power ; while a similar property is not 

 considered to be inherent in the members of the other 

 kingdom. The most recent researches throw, to say the 

 least, considerable doubts on the universal applicability 

 of this test to distinguish animals from plants. Now that 

 the Desmidieas and the Oscillatorieas are, by universal 

 consent, relegated to the vegetable kingdom, and that 

 many bodies described by Ehrenberg as animals are 

 found to be particular stages in the life-history of certain 

 vegetable organisms, this character seems but to follow 

 in the wake of others which have one by one been aban- 

 doned as absolute discriminating tests between the mem- 

 bers of the two kingdoms. Among the more commonly- 

 occurring and familiar movements of vegetable tissues, 

 the dependence of which on external mechanical causes 

 is at present but imperfectly understood, are those mo- 

 tions of the leaves and other parts of plants which are 

 comprised under the common designation of Movements 

 of Sensitiveness or Irritability. It has been well shown 

 by Sachs ' that these movements are of three different 

 kmds, viz. : — • 



I. Those periodic movements which are produced 

 entirely by internal causes, without the co-operation 

 of any considerable external impulse of any kind. Such 

 movements may be terined automatic or spontaneous, 



J "Text-Book of Botany," English edition, Book III., chap. 5. 



