42 THE LAST CRUISE OF THE CARNEGIE 



every breath of air they meet. By following them with a theo- 

 dolite, an instrument for measuring elevation and direction 

 through vertical and horizontal angles, we can study the air- 

 currents at heights up to six or seven miles. Besides the general 

 scientific interest in the movements of the Earth's atmosphere, 

 the aviator will some day come to rely on pilot-charts based on 

 these soundings, just as the mariner relies on wind- and current- 

 charts for the ocean-surface. 



Before leaving the control-room we must glance at the long 

 array of switches, galvanometers, batteries, and ammeters stretched 

 along a table against the starboard wall. Although it is part of 

 the equipment for measuring the elements of the Earth's mag- 

 netic field, some of this apparatus contains small pieces of steel, 

 and must be set up well away from the observatory-domes. One 

 observer sits at this table to control the constant-speed motor 

 for the "marine earth-inductor" which we shall see later. He is 

 in communication through a brass speaking tube with the second 

 observer in the dome. At given signals he records the readings 

 of the ammeters of galvanometers before him. 



In the control-room we also find the Sperry gyroscopic pitch- 

 and-roll recorder. Magnetic measurements at sea are usually 

 affected by small errors caused by rolling, pitching, and scending 

 of the vessel. Though small, these errors are important where 

 accurate determinations are desired of the distribution and of 

 progressive changes in the Earth's magnetism — as on the Carnegie. 

 A study based on records from this instrument has shown that 

 when the vessel heads on any one of the four cardinal points of 

 the compass, no error is introduced into the measurements. A 

 record of the rolling and pitching of the ship during magnetic 

 stations can be studied later at headquarters to detect these 

 disturbing effects. 



We have spent a long time in the cramped quarters of this little 

 room, but one can see that in it lies the central nervous system of 

 the magnetic and oceanographic equipment. A few steps down 

 and we have left the quarter-deck. Standing in the waist of the 

 ship we see curious nets hanging from the whale-boat platforms. 

 These long cones of silk bolting-cloth are used to collect plankton. 



