NATURAL THEOLOGY. 351 



thready is as incontestably mechanical as a wire- 

 drawer's mill. In the body of the silkworm arc 



two bags, remarkable for their form, position, and 

 use. They wind round the intestine ; when drawn 

 out they are ten inches in length, though the ani- 

 mal itself be only two. Within these baffs is col- 

 lected a glue ; and, communicating with the bags, 

 are two paps or outlets, perforated like a grater 

 by a number of small holes. The glue or gum, 

 being passed through these minute apertures, 

 forms hairs of almost imperceptible fineness ; and 

 these hairs, when joined, compose the silk whicia 

 we wind oft' from the cone in which the silkworm 

 has wrapped itself up : in the spider, the web is 

 formed from this thread. In both cases, the ex* 

 tremity of the thread, by means of its adhesive 

 quality, is first attached by the animal to some 

 external hold ; and the end being now fastened to 

 a point, the insect, by turning round its body, or 

 by receding from that point, draws out the thread 

 through the holes above described, by an opera- 

 tion, as hath been observed, exactly similar to the 

 drawing of wire. The thread, like the wire, is 

 formed by the hole through w^hich it passes. In 

 one respect there is a difference. The wire is the 

 metal unaltered, except in figure. In the animal 

 process the nature of the substance is somewhat 



