1 84 ELEMENTARY SCIENCE 



Weather-maps are marked by two kinds of lines. One 

 kind records the variations in temperature over the country; 

 the other records the variations in pressure. This is done 

 by tracing across an ordinary map lines which connect 

 points of equal temperature; or in the case of pressure, 

 lines which connect points of equal pressure. Lines which 

 connect points of equal temperature are called isotherms 

 (see Fig. 67) ; those which connect points of equal pressure, 

 isobars. The fact that isotherms and isobars do not corre- 

 spond shows that other factors as well as heat affect the 

 distribution of pressure. One of these is the fact that 

 water vapor is lighter, per cubic foot, than air. So the 

 more of water vapor there is in air, other things being equal, 

 the lower the pressure. Hence we may expect to find a ten- 

 dency for pressure to be less over the oceans than over 

 the continents, and this expectation is borne out by a 

 computation of the facts shown by Fig. 66. The average 

 pressure of the land areas figures out as more than the 

 average pressure of water areas. But we must remember 

 that the atmosphere is very mobile (movable), and that the 

 earth is constantly spinning from west to east. As it 

 spins, the gaseous outer part of it (atmosphere) does not 

 quite keep up with the solid and fluid inner parts, especially 

 in the equatorial region, where the motion of rotation is 

 much faster than toward the poles. So it does not surprise 

 us to find that areas of low or high pressure show a general 

 tendency to move, and may not lie over the areas where 

 they originated. 



Also we note in Fig. 66 that, as expected, the pressure 

 recorded in the equatorial regions is decidedly less than 

 that to north or south. 



As to temperature, we have noted that as altitude 



