Elasticity of Spider Lines. 193 



the slightest current was sufficient to drive the thread against 

 the scales, from which it could only with difficulty be freed. 



The small weights were gradually fixed to the lower end of 

 the thread, and the corresponding readings of the marks taken. 

 The data were thus obtained for a stress-strain diagram, and 

 are shown on the annexed curve. The abscissae represent 

 percentage stretchings, and the ordinates weights applied. 



The actual stretchings obtained are given below : — 



Amount of 



Percentage 



stretching. 



stretching. 



1*775 centim. 



3-28 



2-775 „ 



5-11 



4-25 



7-85 



5*1 „ 



9-42 



6-325 „ 



11-68 



7-175 „ 



13-25 



W 



eight applied. 



2-0 



milligrammes 



3-5 



•* 



6-5 



•• 



8-5 



** 



11-5 



;y 



13-5 



n 



Original length between the marks, 54' 15 centimetres. 



One milligramme corresponds to 127*3 kilogrammes per 

 square centimetre. 



The breaking-weight of this thread was found to be 17 

 milligrammes, that is, 2*16 x 10 6 grammes per square centi- 

 metre. This number does not differ greatly from those for 

 the breaking- weights of copper (annealed), bronze, drawn 

 gold, palladium, and silver. It is rather greater than that fox- 

 cast iron, but considerably less than steel, and indeed than 

 silk fibre. 



The value for the breaking-weight per unit of cross section 

 of the spider's thread is given on the assumption that the 

 thread is circular. This is not quite correct, however, as it is 

 really composed of four or six strands parallel to each other. 

 Each of these strands, again, is made up of about one 

 thousand exceedingly fine threads. The cross section of the 

 complete thread being, therefore, virtually four or six small 

 circles instead of one large circle, the value given above for 

 the breaking-weight is probably from five to ten per cent, too 

 small. 



The diameter of the thread was very carefully measured 

 for me by Dr. William Snod grass, of the Physiological 

 Laboratory, by means of a very finely-divided micrometer- 

 scale and a powerful microscope. As measured by Dr. Snod- 

 grass, the diameter was found to be almost exactly 10 * 00 of 

 a millimetre. 



One interesting point is at once apparent on looking at the 

 form of the curve. Whereas in all metal threads, whenever 



