Soon we see a few Spiders trotting briskly between the table and the open window. They are running in mid-air. But on what? If the light fall favourably, I manage to see, at moments, behind the tiny animal, a thread resembling a ray of light, which appears for an instant, gleams and disappears. Behind, therefore, there is a mooring, only just perceptible, if you look very carefully; but, in front, towards the window, there is nothing to be seen at all. In vain I examine above, below, at the side; in vain I vary the direction of the eye: I can distinguish no support for the little creature to walk upon. One would think that the beastie were paddling in space. It suggests the idea of a small bird, tied by the leg with a thread and making a flying rush forwards. But, in this case, appearances are deceptive: flight is impossible; the Spider must necessarily have a bridge whereby to cross the intervening space. This bridge, which I cannot see, I can at least destroy. I cleave the air with a ruler in front of the Spider making for the window. That is quite enough: the tiny animal at once ceases to go forward and falls. The invisible foot-plank is broken. My son, young Paul, who is helping me, is astounded at this wave of the magic wand, for not even he, with his fresh, young eyes, is able to see a support ahead for the Spiderling to move along. In the rear, on the other hand, a thread is visible. The difference is easily explained. Every Spider, as she goes, at the same time spins a safety-cord which will guard the rope-walker against the risk of an always possible fall. In the rear, therefore, the thread is of double thickness and can be seen, whereas, in front, it is still single and hardly perceptible to the eye. Obviously, this invisible foot-bridge is not flung out by the animal: it is carried and unrolled by a gust of air. The Epeira, supplied with this line, lets it float freely; and the wind, however softly blowing, bears it along and unwinds it. Even so is the smoke from the bowl of a pipe whirled up in the air. This floating thread has but to touch any object in the neighbourhood and it will remain fixed to it. The suspension-bridge is thrown; and the Spider can set out. The South-American Indians are said to cross the abysses of the Cordilleras in travelling-cradles made of twisted creepers; the little Spider passes through space on the invisible and the imponderable. But to carry the end of the floating thread elsewhither a draught is needed. At this moment, the draught exists between the door of my study and the window, both of which are open. It is so slight that I do not feel its; I only know of it by the smoke from my pipe, curling softly in that direction. Cold air enters from without through the door; warm air escapes from the room through the window. This is the drought that carries the threads with it and enables the Spiders to embark upon their journey.