430 THE POPULAR SCIENCE MONTHLY. 



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 temper- 

 ature is the cause of the motion in the air. 



The shade of a single tree, therefore, cools not only by intercept- 

 ing the sun's rays, but also by the effect of gentle fanning. The shel- 

 ter 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 uncomfort- 

 able 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 Mevieto. 



-*+- 



COUNTING BY THE AID OF THE FINGERS. 



By Prof. JOHN TEOWBEIDGE. 



ONE cannot with any reason contend that the universal possession 

 of ten fino-ers argues a natural tendency of the human mind 

 toward the decimal system ; it is certainly true, however, that multi- 

 tudes of men and women find their fingers of great assistance in 

 arithmetical operations. The intelligent school-teacher is apt to dis- 

 courage 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 



