April 14, 1910] 



NATURE 



203 



lines. Pestilence and plague followed them ; but we 

 can safely say that the share that they had in these results 

 was to disturb the filth of previous years which had 

 accumulated in their channels, and when they ran through 

 undrained villages to scatter the germs of disease far and 

 wide. 



Croydon may be regarded as the centre of a number of 

 bournes. In addition to that known as the Croydon 

 Bourne, there is what was, no doubt, once its tributary 

 bourne, the Merstham or Marlpit Lane Bourne. There 

 was formerly one in the Wickham Valley, and others at 

 Cheam, Carshalton, and Epsom. The Merstham Bourne, 

 which is also flowing this year, yielded, according to Mr. 

 Latham, 1,108,530 gallons a day on ^larch 21 at Stoat's 

 Nest. This now rises in a culvert which was made to 

 receive the waters, when the springs ran high, in the build- 

 ing of Merstham Tunnel, and it follows the valley north- 

 ward towards Croydon, but sinks into the ground, as it 

 has from time immemorial, at Red Lion Green, Smitham 

 Bottom. This disappearance has not yet been explained. 

 If it continued a mile and a half further along the valley 

 it would join the Croydon Bourne at Purley, and together 

 they would proceed by the bourne culvert to swell the 

 volume of the Wandle where it emerges from beneath 

 Croydon. 



The present flow of the Croydon Bourne is not so 

 extensive as that of 1904, when its point of origin gradu- 

 ally receded from the grounds of the " Rose and Crown," 

 W'arlingham, and the gasworks, which became completely 

 surrounded with water, to the Marden Vallev, which it 



surface of an iron or steel bar when permanently deformed, 

 and they indicate the locality of the distortion. Observa- 

 tions show that these lines, in simple cases of loading, 

 have an inclination of about 50° to the direction of maxi- 

 mum principal tension. Assuming that this inclination 

 remains the same under more complex conditions of load- 

 ing, it becomes possible to construct — at least in direction — 

 the system of maximum principal tension from the lines 

 of Liiders found upon a distorted piece. This method has 

 been employed with some success, but it suffers from 

 certain disadvantages. When the variation of stress is 

 considerable, only those parts of the bar where the stress 

 is greatest show any lines, unless the piece is so much 

 deformed that the original distribution of stress is greatly 

 altered. Thus the constructed stress system is either in- 

 complete or inexact. 



A more useful method is suggested by the similarity in 

 the problems of elastic solids and of viscous fluids. Stress 

 lines, which are lines of stress direction in a solid, possess 

 certain p>oints of similarity with stream lines, which are 

 lines of flow direction in a liquid. In order to verify this 

 connection, the comparison has been made between the 

 stream system and the corresponding stress system for a 

 number of two-dimensional cases.' The method employed 

 consists in subjecting a steel bar of uniform thickness, but 

 of varying width, to a tensile load, until a number of 

 Liiders' lines are developed. The corresponding flow 

 diagram is obtained, by means of the well-known Hele- 

 Shaw apparatus, for a channel of the same outline as the 

 bar. The position of one point on each line of Liiders is 



Steel bars 2J inches wide, | inch thick, reduced in width to i\ inches by means of (i) two saw cuts, (■?) two angular notches of 90°, (5) two semicircular 

 notches. The light bands are the lines of Liiders. The fine dark lines have been transferred from the corresponding flow diagrams (2X (4)1 a^ (6)- 



enuered, as far as Bughill Farm. Thence the dry chalk 

 valley winds away to the escarpment, and we may reason- 

 ably conclude that the stream formerly filled the whole 

 valley. This year, although the gasworks at Whyteleafe 

 have a fair amount of water in the field around them, the 

 rising point has fallen short of Bughill Farm, and in the 

 opposite direction has not reached Purley. Its yield was, 

 however, reported by Mr. Latham as 2.300,980 gallons a 

 day on March 21 at a spot below the " Rose and Crown." 

 There is no regularity in the appearance of the bourne. 

 It last was seen in the winter of 1903-4, and previously 

 in 1897. There were annual flows from 1876 to 1883 

 inclusive, and it is interesting to note that Mr. Latham 

 has accurately predicted the last fourteen flows from the 

 rainfall measured in numerous stations on and about the 

 Downs. E. A. M. 



transferred to the flow diagram, and the form of a line 

 of distortion passing through this point is constructed 

 geometrically, assuming that the stream lines represent 

 stress lines, and that the inclination of the lines of dis- 

 tortion to these is 50°. The constructed line is then super- 

 posed upon the corresponding line of Liiders ; any diverg- 

 ence is an indication of a defective method or of faulty 

 construction. 



The figures illustrate the results of three such cases. 

 Although the correspondence between the constructed lines 

 and the actual lines of Liiders is not perfect, due chiefly 

 to certain inherent defects in the method, there seems little 

 doubt that in these cases the stream-line system is a fairly 

 accurate representation of the system of maximum principal 

 tension. G. H. G. 



STRESS LL\ES AND STREAM LINES. 



'T' H E general conditions of stress at any point in a loaded 

 body have been reduced to certain mathematical 

 equations, but there is considerable difficulty in applying 

 these equations to any but the simplest cases. Experi- 

 mental methods, therefore, are welcome, though they may 

 give only approximations to correctness. One such method 

 consists in deducing the stresses from the distortions pro- 

 duced by the load. The application of this method is 

 frequently very difficult, but it has been simplified by the 

 discovery and study of the phenomenon known as Luders' 

 I'mes. - ' • ■ . 



Liiders' lines are markings which appear upon the 

 NO. 21 II, VOL. S.;] 



SCIENTIFIC KNOWLEDGE AND INDUSTRIAL 

 DEVELOPMENT. 



'T'HE prizes and certificates of the successful students of 

 ■*■ the Bath Technical School were distributed by 

 Principal E. H. Griffiths, F.R.S., of the University 

 College, Cardiff, on February 2, when he delivered an 

 address in which our educational system was discussed, 

 and the value of technical education emphasised. We are 

 indebted to the account of the proceedings in the Bath 

 Herald of February 5, and to Pen and Pencil for November, 

 1909, for the facts contained in the following summary. 



1 "A New Experimental Method of Investigating Certain Systems of 

 Stress." By G. H. Gulliver. Proceedings of the Royal Society of Edin- 

 burgh, Session 1909-10, voL xxz., part i. (No. 3). 



