94 U. S. COAST AND GEODETIC SURVEY 
curacy attainable from one month of observations by the use of the tabular values of 
Table 5, the observations for the years 1931 and 1940 at Portland, Maine, may be used. 
For purpose of illustration it will be sufficient to take every other month of each year. 
Correction of monthly range of tide to mean value by tabular factors 
PORTLAND, MAINE 
1931 1940 
Month 
Range Factor Mean range Range Factor Mean range 
Feet Feet Feet Feet 
Jamumanyies sate pe eee See 8. 55 1. 029 8. 80 9. 28 0. 973 9. 03 
IG eC) eee ey ra aE ee 8. 99 1. 029 9. 25 9. 20 0. 972 8. 94 
TA Ug ee aoe Se ce eee 9. O1 1. 029 9. 27 i), Li 0. 972 8. 85 
Jiilivest <2 eben Nes sea Ae meat pee 8. 77 1. 029 9. 02 9. 03 0. 971 8. 77 
September2s2 222522 o ee 8. 56 1. 029 8. 81 9. 38 0. 971 9. 11 
INovembertets 32 %2 2 eteeeee 8. 48 1. 029 8. 73 9. 16 0. 971 8. 89 
The primary determination of mean range at Portland for the 19 year period 
1930-1948 is 8.95 feet. For 1931 the computed mean ranges in the table above differ 
from the primary value by as much as 0.3 foot, and for the year 1940 by very nearly 
0.2 foot. In this connection it must be noted since the months of the moon’s phase, © 
distance, and declination are respectively 29%, 27%, and 27% days, the variations in 
the height of high water from day to day due to changes in the position of the moon 
will be more nearly eliminated in 29 days than in a month of 30 or 31 days. For that 
reason the use of a group of 29 days would give better results than a month of 30 or 31 
days. 
To illustrate the results derived for the same station by the method of comparison 
and that of tabular values it will be instructive to derive the mean range for the months 
of 1931 and 1940 at Portland by the method of comparison using Boston as the primary 
station. For the 19 year period 1930-1948, the mean range at Boston is 9.45 feet. 
The derivation is shown in tabular form below. 
Correction of monthly range of tide to mean value by comparison 
PORTLAND, MAINE 
1931 1940 
Month 
Portland Boston Ratio Mean range| Portland Boston Ratio Mean range 
Feet Feet Feet Feet Feet Feet 
Ae ATVs ee 8. 55 9.138 0. 937 8. 85 9. 28 9. 82 0. 945 8. 93 
IV EtaTic lavage eile Baya 8. 99 9. 51 0. 945 8. 93 9. 20 9. 73 0. 946 8. 94 
Nipper ee osc minen 9. O01 9. 58 0. 941 8. 89 9. 11 9. 62 0. 947 8. 95 
ily ec eis 2 Eee Said 9. 29 0. 944 8. 92 9. 03 9. 52 0. 949 8. 97 
Septemiber-=- 2222555" 8. 56 Oeniel 0. 940 8. 88 9. 38 9. 92 0. 946 8. 94 
Nowa. eet Se 8. 48 9. 00 0. 942 8. 90 9. 16 9. 66 0. 948 8. 96 
The results derived by the method of comparison are seen to be much better than 
those derived by means of the tabular values. The greatest difference from the pri- 
mary value of 8.95 feet is by the former method only 0.1 foot while by the method of 
tabular values the greatest difference was 0.3 foot. In part the superiority of the 
comparison method lies in the fact that since the same number of days is used at both 
