The process of fixing and drawing out a line can be conveniently watched in the case of a Spider imprisoned in a glass vessel, and it will be seen, by the aid of a lens, that a large number of very fine lines starting from the point of attachment seem to merge into a single line as the Spider moves away. This has given rise to the prevalent and very natural idea that the ordinary spider’s line is formed or “woven” of many strands. This, however, is not the case,[272] for the fine attachment-lines are not continued into the main thread, but only serve to anchor it to the starting-point. As has been said, the spider can throw into play a varying number of spinning tubes at will, and in point of fact those used in laying down these foundation-lines are either two or four in number. The spider, however, often finds it necessary to strengthen such a line by going over it afresh. Every one must have noticed that orb-webs frequently bridge over gulfs that are clearly quite impassable to the spider in the ordinary way. They often span streams—and Epeirid spiders cannot swim—or they are stretched between objects unattainable from each other on foot except by a very long and roundabout journey. When this is the case, the animal has had recourse to the aid of the wind. A spider of this family placed on a stick standing upright in a vessel of water is helpless to escape if the experiment be tried in a room free from draughts. With air-currents to aid it, silken streamers will at length find their way across the water and become accidentally entangled in some neighbouring object. When this has happened, the spider hauls the new line taut, and tests its strength by gently pulling at it, and if the result is satisfactory, it proceeds to walk across, hand-over-hand, in an inverted position, carrying with it a second line to strengthen the first. This is exactly what happens in nature when a snare is constructed across chasms otherwise impassable, and it may be imagined that the spider regards as very valuable landed property the foundation lines of such a web, for, if destroyed, the direction or absence of the wind might prevent their renewal for days. They are accordingly made strong by repeated journeys, and are used as the framework of successive snares, till accident at length destroys them.