47 



The heights of the M components thus computed, and for compari- 

 son the recorded heights for the first day's observations are shown 

 in figure 25. 



15- n 



M Components 



Lunar Hours 

 9 le is 



r 

 21 



8.^ 



2^ 



3 6 Q le 15- 18 



Mean Solar Hours 



Figure 25. — M components at Sitkr, Alrska. 



87. Stencils. — The summation of the hourly heights is faciUtated by 

 the ingenious device of cutting stencils which, when laid over the 

 tabulated observed heights on a sheet in standard form, show through 

 the openings the heights to be added to give the sums for each com- 

 ponent hour. Two stencils are prepared for each successive 7-day 

 period shown on a standard sheet, one for the even component hours 

 and one for the odd, lines being drawn on the stencil to connect the 

 observations to be taken for each component hour. 



88. The stencils ordinarily used are prepared for the program of 

 computation illustrated for the M component, in which each observed 

 height enters once and only once in the summations of the 24 com- 

 ponent hours. On the stencils for those components whose component 

 hour, like the lunar hour, is longer than the mean solar hour, a com- 

 ponent hour is repeated at the intervals at which its tabulation must 

 shift back a unit to most nearly correspond to the mean solar hour; 

 and both of the observed values so indicated are included in the sum- 

 mation for the component hour. If the component hour is shorter 

 than a mean solar hour, a component hour is omitted from the stencil at 

 the corresponding intervals at which the tabular values must shift 

 forward a unit. The aggregate of the sums for the 24 component 

 hours taken from stencils in this form may be checked against the 

 sum of all of the observations for the period, but the divisors for com- 

 puting the averages may not be the same as the number of component 



