292 



SCIENCE FROM AN EASY CHAIR 



sp.c 



species, who is much smaller, fortunately for him, can 

 safely run on a hanging line and thus can rapidly escape 

 from the side of his mistress, who, after receiving his 

 caresses, has an unpleasant habit of seizing, killing, and 



sucking the blood of the 

 adventurous male, should 

 he linger longer in her com- 

 pany, and fail in the agility 

 and rapidity of his exit. 



The threads of the gar- 

 den spider (the Porte-croix 

 of the French, white-cross 

 spider, Epeira diadema, Fig. 

 5 1) are fixed by astronomers 

 in their telescopes for the 

 purpose of giving fine lines 

 in the field of view, by which 

 the relative positions of stars 

 may be accurately measured. 



For a century astronomers 

 desired to make use of such 

 lines of the greatest possible 

 fineness, and procured at 

 first silver wire drawn out 

 to the extreme limit of 



FIG. 50. One of the two middle 

 spinnerets of the common garden 

 spider (Epeira diademd), to show 

 the three kinds of spouts or spigots 

 (one thousand in all) corresponding 

 to the three kinds of silk-glands. 

 Each kind of " spigot " pours out a 

 different kind and size of thread. 

 sp.c, one of the big spigots of the 

 cylindrical glands; sp.t, middle- tenuity attainable with that 

 sized spigots belonging to the tree- metal. They also tried hairs 



form glands ; ss and s.ss, the small- (^-th of an inch thick) and 



threads of a silk-worm's 

 cocoon, which are split into 

 two component threads each only ^A^th of an inch 

 thick. But in 1820 an English instrument maker named 

 Troughton introduced the spider's line. This can be 

 readily obtained three or four times smaller in breadth than 

 the silk-worm's thread, and has also advantages in its 



