1916] McEwen: Hydrographic Oh:scr cat ions of Scrippa Institution 257 



A. THE REGION SURVEYED 



1. Number of Observations in Different Portions of the Region 

 Both in hydrographic and biological investigations it is often de- 

 sirable to divide the area to be explored into sections of convenient 

 size and to deal separately with the observations made in each section. 

 Rectangular sections, having bases equal to five minutes of longitude 

 and altitudes equal to five minutes of latitude, have proved very con- 

 s'enient in our oceanic investigations (Michael and McEwen, 1915, 

 p. 46). Accordingly, each number written below the maps (pis. 1-3) 

 is that of the section, twenty-five square minutes in area counted from 

 the base-line, 114^ W, and each number in the column at the left is 

 that of the section north or south, as the case may be, of the other 

 base-line, 32° N. Each of these numbers divided by 12 gives the 

 number of degrees from the corresponding base-line to the center of 

 the section to which it refers. Thus the two numbers corresponding 

 to each section locate its center exactly, and therefore all observations 

 are located to within an error of 2'.5. The position of a section is 

 indicated in the text (Michael and McEwen, 1915, p. 46) by a section 

 number giving the distance tvest of 114° W, accompanied by a sub- 

 script giving the distance north or an exponent giving the distance 

 south of 32° N. Thus 40^ defines the section which is the fortieth west 

 of 114° W and the fourth north of 32° N, while 40^ defines the section 

 which is the fortieth west of 114° W and the fourth soutli of 32° N. 



The number of observations, biological as well as hj'drographic, 

 made in each section j)rior to June, 1915, is approximately represented 

 by stippling (pis. 1-3) ; hence these plates represent graphically the 

 approximate number and distribution of observations throughout the 

 whole area surveyed. Fully half of the observations are hydrographic. 



2. Topography of the Ocean Bottom 



Since it is desirable at the outset to have a general idea of the 

 region explored, attention is first called to the contour maps (pis. 1-3) 

 of the region which extends from Point Conception, 35° N, to Guada- 

 lupe and Cerros islands, 28° N. The contours were determined, for 

 the most part, from soundings entered on the United States Coast and 

 Geodetic Survey charts; but our own soundings (]\Iichael and McEwen, 



