50 THE LAST CRUISE OF THE CARNEGIE 



of the ionization of the atmosphere or of the permanent negative 

 charge of the Earth. 



Intimately connected with the number of ions in the air is its 

 electrical conductivity, or its ability to carry an electric current. 

 It is measured in this laboratory with an automatic photographic 

 recorder. A stream of air is drawn through a duct past a cylinder 

 at its center. A small battery causes a current of one millionth 

 of a millionth of an ampere to pass through the air, and a delicate 

 electrometer measures the air's conductivity. 



The air over the sea is much more free of dust than over land, 

 but the influence of this pollution on the elements of atmospheric 

 electricity is so great that systematic "dust-counts" must be 

 made even far from land. Some years ago, when the volcano 

 Krakotoa erupted, such quantities of dust were blown into the 

 atmosphere that it took two years for it to settle over the Earth. 

 Even in normal years pollution may vary from 1,000,000 particles 

 per cubic inch to a few thousand. When dust is abundant the 

 atmospheric conductivity is decreased and the potential gradient 

 rises to as much as 300 volts per meter. The Aitken counter is 

 used to determine the pollution of the atmosphere. When moist 

 air is suddenly expanded, the water present condenses as droplets, 

 provided some dust-particles are present to act as centers of 

 condensation. In the Aitken counter, the droplets so formed 

 are enumerated and not the dust-particles themselves. Not all 

 dust is visible under a microscope, for it is believed that such 

 particles as salt-spicules, and even aggregates of water or am- 

 monia molecules, may act as condensation-centers. 



In the chart-room under the bridge is the navigational equip- 

 ment including sextants (sixteen of them), barometers, log books, 

 marine charts, and pilot-books. There are six desks where the 

 observers do their computing. Complete sets of graphs, tables, 

 and calculating books are at hand to facilitate the work. These 

 desks are always filled except when a magnetic or oceanographic 

 station is being occupied; for a large part of our duties consisted 

 in preparation of records. Large windows supply plenty of air 

 and light to the men at work. 



In the center of the chart-room stands the "standard compass," 



