1306 
6.4 meters longer than the shortest radius. The compression of the 
meridian €, varies between e + $v and «— Sr. For central Europe, 
i 00 ae santa 
(e) 1 = 295.98 
and for North-America, 2 = 100° 
(ej) 1 == 295.92. 
5. The methods mostly used for the determination of the com- 
pression of the earth are: . 
I. From geodetic measures, 
Il. From the intensity of gravity, 
II]. From the moon’s parallax, 
IV. From the lunar theory. 
By the first method the geodetic measures made in the United 
States of America give | 
EVOL 2°12 lo Soh 4 AO ee 
This agrees within the limits of the mean error with the value 
296.0 found above. 
From a great number of determinations of the intensity of gravity 
HeLMERT derived 
el B08 8 ds lers et eee 
This result agrees with the final result from the American deter- 
minations, viz.: 
LEAD a (ee 
In judging the value of these results it must be remembered that 
both the direction (method I) and the intensity (method IT) of gravity, 
before they are used for the determination of the figure of the 
geoid, or of an ellipsoid of reference, need certain corrections, which 
have been applied by different investigators more or less in agreement 
with the hypothesis of isostasy. All investigators however use 
approximate formulas, and it is not clear which of the definitions, 
treated in art. 2 above, has been adopted. The American investigators 
take a constant depth below the actual surface of the earth (under 
the sea even below the bottom). HeLMERT uses the reduction as in 
free air‘), thus assuming that the isostatic compensation is complete. 
Now it is of course impossible from the observations to decide 
between the three cases of art. 2, and also the corrections computed 
under the three assumptions will be very nearly equal. But small 
1) The American observations reduced by the free air method give instead of 
(II’) e~! = 202.1 + 1.7. See Bowie, Effect of topography and isostatic compensation 
upon the intensity of Gravity, second paper, p. 26. 
