THE CONQUEST OF THE ZONES 



crude apparatus for measuring the height of sun and 

 stars, as the mediaeval navigators are known to have 

 done. 



The simplest and crudest form of measurer of which 

 the record has been preserved is known as the cross- 

 staff. This consisted essentially of a stick about a yard 

 in length, called the staff, on which a cross-piece was 

 arranged at right angles, so adjusted at the center as to 

 slide back and forth on the staff. An eye-piece at 

 one end of the staff was utilized to sight along pro- 

 jections at either end of the cross-piece. If the appara- 

 tus is held so that one of the lines of sight is directed 

 to the horizon, and then the cross-piece slid along the 

 staff until the other line of sight is directed toward the 

 sun or a given star, the angle between the two lines of 

 sight will obviously represent the angle of altitude of 

 the celestial body in question. But the difficulty of 

 using an apparatus which requires two successive ob- 

 servations to be made without shift of position is ob- 

 vious, and it is clear that the information derived from 

 the cross-staff must have been at best very vague — 

 by no means such as would satisfy the modem navigator. 



Even the navigators of the fifteenth century were 

 aware of the deficiencies of the cross-staff and sought 

 to improve upon it. The physicians of Henry the 

 Navigator of Portugal, Roderick and Joseph by name, 

 and another of his advisers, Martin de Bohemia, are 

 credited with inventing, or at least introducing, a 

 much improved apparatus known as the astrolabe. 

 This consists of a circle of metal, arranged to be sus- 

 pended from a ring at the side, so that one of its 



[19] 



