48 THE LAST CRUISE OF THE CARNEGIE 



ship. By taking the mean of several hundred such readings he 

 has made an accurate measurement from which the decHnation 

 may be computed. 



This instrument was designed by Peters and Fleming of the 

 Department of Terrestrial Magnetism, and was made in its shop. 

 The method is superior to older methods used at sea which de- 

 pended on hasty readings taken as the Sun's image, or a shadow, 

 flits across a moving compass-card on a rolling ship. Three ob- 

 servers are required to take a declination-measurement. One 

 man's duty has been described. A second reads the altitude of 

 the Sun from time to time, for it seldom happens that weather- 

 conditions are perfect exactly at sunrise or sunset, and corrections 

 for altitude must be applied. The third observer is the recorder. 

 He must be a sleight of hand artist, because he had to write down 

 the readings of the other two and keep a second-to-second record 

 of the time when each of these is made. 



On the starboard wing of the bridge is located an apparatus 

 for collecting the radioactive materials in the atmosphere, which 

 are present in only infinitesimal amounts. When a measured 

 volume of air is drawn through the collector over negatively 

 charged metal foil, the desired particles are deposited on the foil 

 because they carry a positive charge. Let us now follow the 

 observer into the atmospheric-electric laboratory, where he will 

 measure the amount of radioactive material collected. This 

 electric laboratory is located just abaft the bridge, directly amid- 

 ships. It is entered from the foot of the steps leading to the 

 bridge. The observer places the metal foil in an ionization- 

 chamber where the rate at which the radioactive material produces 

 electrified particles or ions is measured. This rate is a direct 

 measure of the amount of radioactive material collected. 



Another instrument counts the ions normally present in the 

 atmosphere, by extracting them from a measured volume of 

 air. There are usually about 30,000 of these per cubic inch, but 

 their origin is unknown. Under the action of the Earth's electric 

 field, positive ions are traveling toward the Earth and negative 

 ions upward into the air, giving rise to an air-earth electric cur- 

 rent which makes no impression on our senses. The rate at 



