THE EQUIPMENT 



47 



collimating-compass, the card ordinarily viewed from above is 

 replaced by a set of vertical scales which may be seen by looking 

 horizontally through openings in the sides of the compass-bowl. 

 An observer brings the image of the rising Sun, let us say, to one 



The Wenxer Salinity-bridge 

 An apparatus giving the salt-content in a sample of sea-water by measuring the resist- 

 ance it offers to the passage of an electric current. 



of these vertical scales with an ordinary sextant and measures 

 the horizontal angle between them. With the Sun's image on 

 the vertical scale he can make continuous readings of its posi- 

 tion, as the compass swings back and forth with the roll of the 



