158 U. S. COAST AND GEODETIC SURVEY. 



The principal hjdrographic datums in general use are as follows: 

 Mean low water for the Atlantic and Gulf coasts of the United States 

 and Porto Rico. Mean lower low water for the Pacific coast of the 

 United States, Canada, and Alaska, and the Hawaiian and Philip- 

 pine Islands. Approximate low water springs for the rest of the 

 world, with a few exceptions. 



For use on the predicting machine the datum must be defined by 

 its relation to the mean sea level, and this relation is usually deter- 

 mined from a reduction of the Ifigh and low waters. 



Column A of Form 444 is designed for the differences by which 

 the epochs of the components are adapted once for all for use with 

 the unmodified Greenwich ( V^ + u) 's of each year. These differences 

 take account of the longitude of the station and also of the time 

 meridian used for the predictions, and are computed by the formula 



K'-K = pL-j^ (492) 



in which 



k' — k = adapted epoch — true epoch. 



29 = subscript of component, which indicates number of periods 

 in one component day. For the long-period components 

 Mm, Ssa, Sa, Msf, and Mf, p should be taken as zero. 



L = longitude of station in degrees ; -f- if west, — if east. 



a = speed of component in degrees per solar hours. 



6' = longitude of time meridian in degrees; + if west, — if east. 



The values of the products ^v for the principal time meridians 



may be taken from Table 35. For any time meridian not given in 

 the table the products may be obtained by direct multiplication, 

 taking the values for the component speeds (a) from Table 3. 



Column B is designed for the reduction of the amplitudes to the 

 working scale of the machine. The scale is unity when the 40-foot 

 height dial is used, 2 for the 20-foot height dial, 4 for the 10-foot 

 height dial, and 10 for a 4-foot height dial. The working scale 

 should be entered at the head of the column and used as a factor 

 with the amplitudes in order to obtain the values for this column. 



Columns C and D are designed to contain the adapted epochs in 

 positive and negative forms which may be used additively with the 

 Greenwich (Fo + u)'s. It will be found most convenient to compute 

 column D, first, by applying the difference in column A to the k in 

 the preceding column and entering the result with the negative sign. 

 If the direct apj)lication of the difference should give a negative 

 result, this must be subtracted from 360° before entering in column D. 

 The values for column C may then be obtained by applying 360° 

 to the negative values in column D. 



Form 445, settings for tide-computing machine {jig. 34) • — This form 

 is designed for the computations of the settings for the predicting 

 machine for the beginning of each year of predictions. The forms 

 are bound in books, a separate book being used for each year of 

 predictions. This form is used in connection with Form 444, and 

 lor convenience the order of arrangement of the components is 

 identical in the two forms. The name of the station, the time merid- 



