MEASUREMENT OF FUTURE MOVEMENTS ON SAN ANDREAS FAULT. 157 
The plates, spindles, and caps are the same as those described under “ Description of 
Monuments” at Olema. The distances between these stations being too great to observe 
upon the spindles with advantage, pencils were wrapt with white cloth and set in the spindle 
cups upon the plates, the bronze protecting caps being used for background, as in the Olema 
measurements. 
Base-line measures. — The stakes, made long enough to stand above the grass, were lined 
in with the alt-azimuth instrument, and to avoid the possibility of errors they were all 
numbered on top with a blue crayon. The stakes were made of redwood, and the method 
of marking was to stick a pin straight down in the top of each at the marking edge of the 
tape. The tape used was marked to hundredths of feet the entire length, the thousandths 
being estimated. The measurements were so taken as to avoid estimating the thousandths, 
excepting on the last measure, the mark being arbitrarily placed at the nearest convenient 
tenth of a foot on the top of each forward stake. 
As the diameter of the pins used was almost exactly the same as the width of the 0.1-foot 
marks on the tape, the marking could be done with exceptional accuracy, especially by 
holding the eye directly over the mark in such a way that there would be no parallax. The 
spring balance was fastened to the forward end of the tape, and steadied by means of a cord 
looped so as to slip up and down on a pole, held by a man who at the same time watched 
the tension. To avoid any pulling against the stakes, the height of the tape was regulated 
by means of the loop so as just to graze the top of the stake. All the marking was done 
at the forward end of the tape, the officer in charge at the rear end simply steadying on the 
mark of the previous measure and then reading the tape. 
* The lengths of the base joining the southeast and northwest monument resulting from the 
measurements are the following: 
FEET. 
inet aneASUre eee were ee Ree Wee Beek Eee Ee sls be Sole SO20 
ECON TLOGSUT CHEM CE Wn gt cada eae ey ae = os | OOL 1 9SS 
Mean nee eee. tt ee es et” 87-800 
Relative elevations. — By means of precise leveling the relative elevations of the four 
fixed monuments, taking the center of the top of the bronze plate in each case, were 
found to be as follows: 
FEET. 
Nortinwest morument. mals) ses oe we. aoe 9 (assumed)'50.000 
Northeast monument . ; bade reed le ours. ele i. HT st HOO La 
OU SNeAetaIONLITen Tate ie eee rn es ee ee ee a IR 
SOUtMWwestINOnUIMCH tawny OEE — Skccent fa, on gf ee 46 113 
Method of observing angles. — The instrument used was a 10-inch alt-azimuth theodolite, 
carrying two micrometers 180° apart. Each micrometer head is divided to represent sec- 
onds of arc, enabling the observer to estimate to tenths of seconds of arc at each reading. 
In taking the observations, each micrometer was read to correspond with two consecutive 
5-minute divisions, one being back of the referencé mark and one in front. The correc- 
tions for “run” at each station were based upon the observations themselves, the mean of 
all observations at the first two monuments being taken both at Olema and at Crystal 
Springs. In order to eliminate all possible instrumental errors, the observations were, in 
general, taken in four sets, having for the initial reading of each set, 0°, 90°, 45°, and 135°, 
respectively; making for the reversal of the telescope, without changing the setting of the 
circle, the corresponding readings of 180°, 270°, 225°, and 315°. 
Thus, upon each station there were eight pointings of the instrument, representing eight 
portions of the circle equally divided. Since for each of these pointings there are two 
micrometers, each giving two readings of the thread, there were in reality 32 micrometer 
readings for each observed station. The above statements apply fully at Crystal Springs, 
but at Olema one micrometer was not in condition to use, so that the Olema observations, 
while constituting the same number of telescope pointings, represent for each observed sta- 
tion but 16 micrometer readings. 
At Olema, on account of the very small distances between the monuments, large changes 
of focus were necessary for the different pointings. This, combined with the large differ- 
ences of elevation, and lack of perfect centering of the instrument on the plates to correspond 
with the positions of the spindles, prevented the triangles from closing to a very high degree 
of accuracy. Still, when these discrepancies are reduced to errors of distance, they become 
practically inappreciable. 
