Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists . earlygeophysical00soci Year: 1947 NEW INSTRUMENT FOR GRAVITY MEASUREMENTS 27 the instrument and subtract the effect of elevation, which is one part in ten million for every foot. These differences in elevation can be determined to within three or four feet with the Paulin Altimeter, provided that it is used according to a particular system that has been worked out for this purpose, but if greater accuracy is desired the land must be surveyed. These facts make the use of the gravity balance m
Early geophysical papers of the Early geophysical papers of the Society of Exploration Geophysicists . earlygeophysical00soci Year: 1947 NEW INSTRUMENT FOR GRAVITY MEASUREMENTS 27 the instrument and subtract the effect of elevation, which is one part in ten million for every foot. These differences in elevation can be determined to within three or four feet with the Paulin Altimeter, provided that it is used according to a particular system that has been worked out for this purpose, but if greater accuracy is desired the land must be surveyed. These facts make the use of the gravity balance much simpler than any other geophysical instrument except the magnetometer. Recent field work has shown that in reasonably good weather it is easy to make three stations per hour. The torsion balance makes three stations per day and the Government pendulum apparatus three or four stations per month. This measurement which takes only a few minutes appears to have four or five times the accuracy of a single run of the pendulum which takes twelve hours. The information obtained is exactly the same as with the pen- dulum, the total intensity of the gravitational field. For this reason the pres- ence of a heavy mass near by on the surface or at small depth has no effect on the reading; the presence of such a mass merely alters the direction of the Fig. 4. Gradient and gravity curves for assumed structure. resultant force and as the instrument is set exactly in line with this direction the measurement is the same. These surface masses make so much trouble for the torsion balance operator that it seems to be hard for him to believe that they do not disturb this instrument. The relation between the gravity anomaly, which is measured directly by the gravity balance, and the gravity gradient, which is measured by the torsion balance, is very clearly shown in a diagram in Ambronn's Elements of Geophysics, which he has taken from a paper by von Eotvos. Figure 4 is derived from this diagr
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