MEASUREMENT OF FUTURE MOVEMENTS ON SAN ANDREAS FAULT. 155 
The relative elevations of the four monuments, taking the center of the plate in each 
case, are as follows: 
FEET. 
Southwest monument. ....-:- .. .. . . (assumed) 10.000 — - 
Northwest monument . . ree een Sie SA Pe ER 28.005 
INGHINEASUEINON MING tale wl s Br REE ENS et 5 of . OPIS TE 
SOucieastmNonuinentin an seem nen en os os ox «ows oe 61-390 
Base-line measures. — B. A. Baird in charge, reading rear end of tape and recording. 
R. 8. Badger, forward end of tape and reading thermometer. Charles Evans (laborer), 
steadying spring balance attached to end of tape and watching tension of 10.5 Ibs. The 
tape used, a 100;foot steel tape, G. M. Eddy and Co., Catalogue No. 703; was stamped on 
reel “No. 1” for identification in future use. Its width is 0.272 inch; its thickness 0.010 
inch; and its weight per foot 3.8324 grams or 0.00845 lb. This tape, on May 1, was com- 
pared with the standard tape at the University of California, a long level stretch on the 
“bleachers’”’ being used for the purpose. The standard tension of 10.5 lbs. was adopted, 
and no difference in the lengths of the tapes could be detected. 
The standard tape, N.B.S. No. 8, is marked only at zero and 100 feet. The comparisons 
were made between these marks, and the equality of zero to 50 feet and 50 to 100 feet was 
measured on the tape used in the base-measures, there being no measurable difference. 
The constants of the standard tape, N.B.S. No. 8, are: Temperature of observation, 
64.6° F.; Tape supported thruout entire length; tension, 10.5 lbs. avoirdupois; resulting 
values of spaces at 62° F., assuming coefficient of expansion = 0.0000063 per degree IF’. are 
zero to 100 feet = 100 feet 0.00 inch. 
FORMULZ AND CONSTANTS USED IN BASE-LINE COMPUTATIONS. 

Correction for Level = — - — aa) > where 
h = difference of elevation of ends of tape. 
d = distance between supports. 
Correction for Temperature = —l (7'—t)e, where 
1 =length of line corrected for. 
T = standard temperature = 62° F. 
¢ = mean temperature of tape. 
e = coefficient of expansion = 0.0000063 per degree F. 
2 
Correction for Sag =-5 . (2) , where 
w = weight of tape per foot = 0.00845 lb. per foot. 
P = standard tension of 10.5 lbs., the same as used in measures. 
d and I same as above. 
1 /w\? 1 /0.00845\? 
— . = 2700. 
From the above, 54 ) DA ( 10.5 ) 0.00000002700 
The correction for pull, accounting for elasticity of tape, is not necessary, since the stand- 
ard tension was used in the measures. 
Level correction. — In taking the measurements, the center of the spindle, firmly screwed 
into the cup on the bronze plate, as shown in the diagram, fig. 47, was the reference mark on 
the monuments. 

The ordinary correction for level, a is not sufficiently accurate when the differences are 
large, and a second correction has been allowed for. In the corrections at Crystal Springs, 
even third approximations are necessary. 
The computed values of the measured lines are summarized as follows: 
[Computed lengths of bases (feet) .] 


FoRWARD. BACKWARD. MEAN. 
INGWes too. We Ong 6) 5 oss 110.1528 110.5115 a0 Maye es 
NEWs to Ss: BeiMon. 2) = es . 120.5890 120.5878 120.588 
SApeatorSsW A Monss es est +: 79.3728 79.3725 79.373 

