I get rid of it by closing both apertures and I break off any communication by passing my ruler between the window and the table. Henceforth, in the motionless atmosphere, there are no departures. The current of air is missing, the skeins are not unwound and migration becomes impossible. It is soon resumed, but in a direction whereof I never dreamt. The hot sun is beating on a certain part of the floor. At this spot, which is warmer than the rest, a column of lighter, ascending air is generated. If this column catch the threads, my Spiders ought to rise to the ceiling of the room. The curious ascent does, in fact, take place. Unfortunately, my troop, which has been greatly reduced by the number of departures through the window, does not lend itself to prolonged experiment. We must begin again. The next morning, on the same yucca, I gather the second family, as numerous as the first. Yesterday's preparations are repeated. My legion of Spiders first weaves a divergent framework between the top of the brushwood placed at the emigrants' disposal and the edge of the table. Five or six hundred wee beasties swarm all over this work-yard. While this little world is busily fussing, making its arrangements for departure, I make my own. Every aperture in the room is closed, so as to obtain as calm an atmosphere as possible. A small chafing-dish is lit at the foot of the table. My hands cannot feel the heat of it at the level of the web whereon my Spiders are weaving. This is the very modest fire which, with its column of rising air, shall unwind the threads and carry them on high. Let us first enquire the direction and strength of the current. Dandelion- plumes, made lighter by the removal of their seeds, serve as my guides. Released above the chafing-dish, on the level of the table, they float slowly upwards and, for the most part, reach the ceiling. The emigrants' lines should rise in the same way and even better. The thing is done: with the aid of nothing that is visible to the three of us looking on, a Spider makes her ascent. She ambles with her eight legs through the air; she mounts, gently swaying. The others, in ever- increasing numbers, follow, sometimes by different roads, sometimes by the same road. Any one who did not possess the secret would stand amazed at this magic ascent without a ladder. In a few minutes, most of them are up, clinging to the ceiling. Not all of them reach it. I see some who, on attaining a certain height, cease to go up and even lose ground, although moving their legs forward with all the nimbleness of which they are capable. The more they struggle upwards, the faster they come down. This drifting, which neutralizes the distance covered and even converts it into a retrogression, is easily explained.