Page:Popular Science Monthly Volume 3.djvu/681

Rh we see come out a thread of silk. This is at first cast out in any direction, and forms a collection of cords destined to fix the cocoon that is to be spun. Soon the work becomes regular, and the form of the cocoon is outlined. For some hours we can see the worker performing his task across the transparent gauze with which he surrounds himself. By little and little, this gauze thickens, and grows opaque and firm; finally it becomes a cocoon like these I place before you. At the end of about 72 hours the work is done.

Once it has given out its first bit of silk, a worm in good health never stops, and the thread continues without interruption from one end to the other. You see that the cocoon is in reality a ball wound from the outside inward. The thread which forms this ball is 11 miles in length; its thickness is only $1/2400$ of an inch. It is so light that 28 miles of it weigh only 15$1/3$ grains. So that 2$1/5$ lbs. of silk is more than 2,700 miles long.

Let me insist a moment on the prodigious activity of the silk-worm while weaving his cocoon. To dispose of its silk when spinning, it moves its head in all directions, and each movement is about one-sixth of an inch. As we know the length of the thread, we can calculate how many movements are made in disposing of the silk in 72 hours. We find in this way that a silk-worm makes nearly 300,000 motions in 24 hours, or 4,166 an hour, or 69 per minute. You see that our insect yields not in activity to any weaver; but we must add that it is beaten by the marvellous machines that the industry of our day has produced.

All cocoons are not alike. There exist, in fact, different races of silk-worms, as we have different races of dogs. These differences are less obvious in the animals themselves; they are best seen in the cocoons, which may be either white, yellow, green, or gray; some are round, others oval or depressed in the middle (Figs. 10 and 11). The silk of one is very fine and very strong, that of others is coarse and easily broken. Hence their very different values.

All I have said applies to the silk-worm properly so called—to the silk-worm which feeds on the leaves of the mulberry-tree, the Bombyx mori of naturalists. But, some years since, there were introduced into France new species of caterpillars that produce cocoons, and