November 19, 1915] 



SCIENCE 



713 



effect, but this would probably be very slight 

 at points on the open coast and would no 

 doubt be constant and not show in the series 

 of tidal results. 



The following table gives the yearly aver- 

 ages for the tidal stations at the Presidio, San 

 Francisco, just inside the Golden Gate. 



The mean sea level for 16 years (1898 to 

 1913) equals 8.519 feet on the staff. The staff 

 was frequently referred to a substantial bench 

 mark near the tidal station and corrections 

 were applied to take account of any variation 

 in the elevation of the zero of the staff referred 

 to this bench mark. 



The total range during the 16 years was 

 0.36 foot. The greatest difference from the 

 mean is 0.22 foot, while the average difference 

 is 0.0Y5 foot. It is seen that the mean value 

 for any three consecutive years does not differ 

 from the mean for the sixteen years by more 

 than .110 foot. There are 14 3-year groups 

 with an average difference from the mean of 

 0.04 foot, about 1.3 centimeters. 



Ordinarily when a tidal station is estab- 

 lished solely for determining mean sea level 

 from which to extend a line of precise levels 

 a series of tidal observations extending 

 through only three years is made. Judging 

 from the San Francisco records, we may there- 

 fore expect an uncertainty of the plane of 0.04 

 foot or 0.013 meter. 



Whether or not the mean sea levels at differ- 

 ent parts of the same ocean and of different 

 oceans lie in the same equipotential surface is 

 a question which has not been solved in the 

 TJnited States. It is true that the several 

 transcontinental lines of levels indicate that 

 the Pacific is higher than the Atlantic and 

 Gulf, but this may be due to accumulated 

 errors in the thousands of miles of levels in- 

 volved. The results of careful leveling across 

 the Isthmus of Panama show that the mean 



sea levels of the Atlantic and Pacific are in 

 the same equipotential surface within 17.8 

 centimeters. This difference may-be largely 

 due to the unavoidable errors in the leveling 

 and the determination of the sea levels of the 

 two oceans. The mean sea level on the Pacific 

 was determined by observations through only 

 one year and may be in error several tenths of 

 a foot. 



The leveling across Florida is not strong 

 and its results are not conclusive. There are 

 four lines between St. Augustine on the At- 

 lantic and Cedar Keys on the Gulf, with a 

 total range of 0.85 meter in the difference in 

 elevation of the two places. At each of those 

 points tidal observations are being made and 

 within a year or two a new line of levels will 

 be run between them. "We hope to obtain then 

 some definite data in regard to the relative 

 elevations of the two bodies of water. 



In the absence of conclusive information we 

 have assumed in our level net adjustment that 

 the surfaces of the three great bodies of water 

 touching this country are in the same equi- 

 potential surface and the starting points of 

 the various lines of levels are consequently 

 given zero elevations. 



There are two ways in which the errors of 

 leveling show; one in the closing of circuits, 

 and the other in the disagreement in the dif- 

 ference in elevation between each two consecu- 

 tive bench marks as determined by the two 

 runnings of the line between them. We shall 

 not have time to consider at length the closing 

 errors. The closing errors of the circuits of 

 levels run entirely with the new instruments 

 and methods^ are seldom greater than 0.20 

 millimeter per kilometer. This is a clear indi- 

 cation that the accumulative errors in a long 

 line are small. 



The following table shows the principal facts 

 in regard to the closing errors of the 84 cir- 

 cuits forming the net which was adjusted in 

 1912. 



There are many sources of error in leveling, 

 of a systematic nature which may be made to 



2 See the following C. and G. S. publications : 

 Appendix No. 8, Eeport for 1903, and Special 

 Publications Nos. 18 and 22. 



