712 



SCIENCE 



[N. S. Vol. XLII. No. 1090 



you in your various fields of that which. T 

 have called the larger background of knowl- 

 edge. It is only by this that we can see 

 things in their true perspective. Our re- 

 spective sciences and our special fields of 

 research become of value only when their 

 wider relations are apprehended. And 

 may I without unduly magnifying mine 

 office as a biologist call your attention to 

 the fact that biology has contributed no 

 unworthy share to the means of progress 

 in the sister sciences. The contributions 

 of biologists, especially the workers in 

 physiological chemistry, to the general ad- 

 vance of chemical science does not require 

 to be mentioned ; nor do I need to refer to 

 the usefulness to physical chemistry of the 

 fertile ideas of Pf effer and De Vries in the 

 explanation of osmotic pressure. The 

 physicists do not need to be told that by 

 far the most sensitive galvanometer for the 

 measurement of minute currents of short 

 duration is the device of the physiologist 

 Einthoven, designed primarily for use in 

 the study of living organs. The engineers 

 will recall that the method of recording 

 progressive changes on a revolving drum 

 is the application of the kymographion in- 

 vented by Ludwig for the recording of 

 blood pressures; but now employed in se- 

 curing graphic records of a great variety 

 of natural phenomena. 



Or let me reverse the picture and remind 

 you that the physiologist, the pathologist 

 and the physician are laboring to apply the 

 results of your researches in the explana- 

 tion of the normal life processes, and to use 

 them in the discovery of the causes of pain 

 and suffering and disease, to the end that 

 these causes may be overcome. Toward 

 this result all lines of scientific effort are 

 contributory. 



Samxiel S. MaxwelIj 



XJniveesity of Califoenia 



TSE EBJBOBS IN PEECISE LEVELING DUE 



TO IBBEGVLAB ATMOSFHEBIC 



BEFBACTIONS-^ 



Vert accurate determinations of elevations 

 above some datum have been made possible by 

 the great improvements in the wye or spirit 

 level which have taken place during the last 

 half century. In 186Y the International Geo- 

 detic Association defined precise leveling as 

 that which has a probable accidental error of 

 not more than 3 mm. per kilometer. The level- 

 ing run to establish the controlling or funda- 

 mental elevations in the interior of the coun- 

 tries, during the decades which followed, 

 showed these limits to be too liberal. In 1912 

 the International Geodetic Association adopted 

 a resolution calling for a still higher grade of 

 leveling called "leveling of high precision." 

 This is defined as leveling in which every line, 

 set of lines or net is run twice in opposite di- 

 rections on different dates, as far as possible, 

 and whose errors, computed by prescribed 

 formulas, do not exceed =t 1 mm. per kilometer 

 for the probable accidental error and ± 0.2 

 mm. per kilometer for the probable systematic 

 error. 



This class of leveling is easily secured with 

 the modern instruments and methods. In fact 

 the greater portion of the leveling done with 

 the older instruments and methods in the 

 United States by the Coast and Geodetic Sur- 

 vey and by other organizations came within 

 these limits. 



The datum or plane of reference which has 

 been adopted in this and in other countries is 

 mean sea level, that is the surface of the 

 oceans with the water assumed to be at rest 

 and affected only by gravity. This surface 

 may be determined with relation to fixed 

 points on land by long series of tidal observa- 

 tions. The mean surface varies in height from 

 day to day, month to month, and even from 

 year to year. Whether there are secular 

 changes is not definitely known. The disturb- 

 ing influences are the sun and moon, prevailing 

 winds and varying atmospheric pressures. 

 The configuration of the shore may have some 



1 Eead before the WasMngton Philosophical So- 

 ciety, March 13, 1915. 



