The sketch (Fig. 7) shows a device consisting of a spirit lamp and a box of ice. The lamp heats the metallic plate, expands the air which rises and is cooled by convection on coming in contact with the top plate, and descends as shown. However, a fire is not necessary to accomplish this result; it is taking place all over the earth, all the time. A great number of plants depend upon a rising current of air to transport their seeds to distant places. Seeds of the thistle and dandelion variety are sometimes able to travel hundreds of miles, to the great vexation of farmers; and there is a certain class of small spider known as “Balloon Spiders” which also depend upon a rising current of air to carry them from the place of their birth to some distant part where they, of course, hope to start a colony. When I was a boy of eight, I noticed small spiders webbing down from the sky. I was greatly puzzled; it appeared to me that they had attached their web to some stationary object high in the air and were spinning a web in order to lower themselves to the earth. What could that stationary object be? As the sky was clear, I was quite unable to understand this phenomenon, but afterwards I learned from scientific books that there was a class of spiders that managed to rise high in the air by the aid of the wind. It appears that they climb a high tree until they have reached the uppermost extremity and then, from a leaf or twig that projects into the air, they wait for an ascending current of air. Although the spider is exceedingly small--the size of a pin’s head--it has about 200 spinnerets, its ordinary web being formed of no less than that number of extremely fine threads. These are spun out singly into the air until an almost invisible mass of fine webs interlacing each other in all directions and forming an approximately cylindrical network about half an inch in diameter and 18 inches long is produced. Whenever an upward draft of air approximately vertical occurs, it takes this weightless tangle of fine webs with it, and so soon as the spider finds there is sufficient pull to lift its weight, it lets go and ascends with the air. When the Nulli Secundus ascended at Farnborough and landed at the Crystal Palace, Mr. Cody, who was on board, reported what he supposed to be a very curious and unaccountable phenomenon. The balloon was covered with many thousands of minute spiders that it had picked up in the air on the voyage. Certainly this of itself is very strong evidence of the existence of these ascending currents of air. [Illustration: Fig. 8.--_a_ represents a kite in a horizontal wind, _e_, _e_, _e_; _b_, the same kite in a rising column of air, the wind blowing in the direction shown at _f_, _f_. If the kite is a good one, it may pass over to the point _c_.]