BICYCLE T7RES gt 



pie rules, the complexity of the weather map will disappear 

 and a glance at the map will give one information concern- 

 ing general weather conditions just as a glance at the ther- 

 mometer in the morning will give some indication of the 

 probable temperature of the day. (See Laboratory Manual.) 



On the weather map solid lines represent isobars and 

 dotted lines represent isotherms. The direction of the wind 

 at any point is indicated by an arrow which flies with the 

 wind ; and the state of the weather clear, partly cloudy, 

 cloudy, rain, snow, etc. is indicated by symbols. 



84. Bicycle Tires. We know very well that we cannot put 

 more than a certain amount of water in a tube, but we know 

 equally well that the amount of air which can be pumped 

 into a bicycle or automobile tire depends largely upon our 

 muscular energy. A gallon of water remains a gallon of 

 water and requires a perfectly definite amount of space, but 

 air can be compressed and compressed, and made to occupy 

 less and less space. While it is true that air is easily com- 

 pressed, it is also true that air is elastic and capable of very 

 rapid and easy expansion. If a puncture occurs in a tire, the 

 compressed air escapes very 

 quickly; that is, the com- 

 pressed air within the tube 

 has taken the first opportu- 

 nity offered for expansion. 



The fact that air is elastic 

 has added materially to the 

 comfort of the world. Trans- 

 portation by bicycles and au- 

 tomobiles has been greatly FIG. 51. - By squeezing the bulb, air is 



' forced out of the nozzle. 



facilitated by the use of air 



tires. In many hospitals, air mattresses are used in place of 



hair, feather, or cotton mattresses, and in this way the bed is 



