3J: CONSTRUCTION OF ISOBARIC CHARTS 



liowever, g be expressed in feet/second^ units as is customarily done, then we find 

 that in order to raise one pound a vertical distance of one foot the expression 



« .„. „-« pound X mile^ 



0.464 876 X 0- X ^ ^r i 



hour*' 



represents the amount of work which must be performed. Therefore every foot of 



vertical distance will be intersected by 0.464 876 X g level surfaces of gravity. At the 



feet 

 Equator, Avhere gravity equals 32.089 ,, there will be 0.464 876 X 32.089 = 14.917 



such planes ; and at either pole, where gravity equals 32.256, there will be 0.464 876 X 

 32.256=14.995 such planes to eveiy foot of vertical rise. These figures hold true 

 near sealevel, while at greater heights the level surfaces will lie somewhat farther 

 apart. The level surfaces are thus seen to constitute closed surfaces at approximately 

 (jne-fifteenth foot intervals from one another, enclosing the earth and showing a polar 

 flattening similar to that of the ocean surface. 



In order to distinguish the individual surfaces of this system they are numbered 

 as follows : sealevel is numbered zero (0) ; the plane standing about one-fifteenth 

 foot above zero is numbered one (1) ; the plane standing about two-fifteenths foot 

 above zero is numbered two (2) and so onward. Thus the surface numbered ten (10) 

 has an elevation of about two-thirds foot ; number 100 an elevation of about 61 feet ; 

 the planes numbered 1 000, 10 00(3, lOO 000, etc., have respectively heights of about 

 67, 669, 6 690 feet, etc., above sealevel. The true elevations above sealevel of these 

 level surfaces are somewhat greater at the Equator and somewhat less at the poles, 

 than the average values here given. 



If now these level surfaces of gravity are to be used as coordinate surfaces in the 

 atmosphere instead of the surfaces of equal elevation above sealevel, then instead of 

 expressing the elevation of any point in feet above sealevel we must state the ordinal 

 number of the level surface in which it lies. The transformation from " feet aliove 

 sealevel" to tlie ordinal number of the level surface of gravity may be easily per- 

 formed by means of a table showing the relation between the two numbers. Such a 

 table should be calculated for every locality where the elevations of kites, balloons or 

 clouds are measured, and in the following paragraphs I show how such a table may 

 be calculated. 



Designate the elevation above sealevel of the point by ~, and the ordinal number 

 of the level surface in which it lies by V. Then V is equal to the number of level 

 surfaces included between the given point and sealevel. V also expresses the work 

 required to be done in order to raise a unit mass from sealevel to the position of the 

 given point, for it always requires one unit of work to raise a unit mass from one sur- 



