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Page:Popular Science Monthly Volume 12.djvu/446

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the shade extends the air is cooler than in the sun; layers of air of unequal warmth are of different gravity, and this difference of temperature is the cause of the motion in the air.

The shade of a single tree, therefore, cools not only by intercepting the sun's rays, but also by the effect of gentle fanning. The shelter of a thick wood, however, is much more agreeable than that of a single tree. The air in a wood is cooler than that of an open space exposed to the sun. The air from outside is drawn into the wood, is cooled by it, and cools us again. And it is not only the air that cools us, but the trees themselves. Observation has shown that the trunks of trees in a wood breast-high, even at the hottest time of day, are 5° Centigrade cooler than the air. We therefore lose considerable heat by radiation to these cooler objects, and can cool ourselves more easily at a temperature of 25° Centigrade in a wood than at a much lower temperature in an open space. When the objects around us are as warm as ourselves we lose nothing by radiation; what is radiated from us is radiated back by them. This is why we are so uncomfortable in heated and overcrowded rooms. It is generally set down to bad air, and this does certainly contribute to it, but it is chiefly the result of disturbed distribution of heat, as has been plainly shown by experiments on the composition of such air, which makes many people feel ill.—Contemporary Review.



ONE cannot with any reason contend that the universal possession of ten fingers argues a natural tendency of the human mind toward the decimal system; it is certainly true, however, that multitudes of men and women find their fingers of great assistance in arithmetical operations. The intelligent school-teacher is apt to discourage the pupil's use of the fingers in addition, and to encourage mental counting without their aid. I have been interested to discover the nature of this mental process which goes on apparently without the aid of the hands. From questioning a large number of persons, I find that five or six is the limit to the numbers of things which one can repeat, and also keep the count. Of course, this limit can be much exceeded by practice; one person who was interrogated could count up to fifty, but he was an astronomer. Most persons reply to the interrogatory, "How do you keep the count?" by saying, "I run up to five, and then again to five, and so on." In most cases it was found that a subdivision into ones and twos preceded this division into fives. The division into twos seemed to be the most common; by