NAVIGATION OURTISS. 131 



At this point two most vital discoveries making for advancement in 

 navigation were made. The rise of modern navigation may fairly be 

 dated from the invention of the sextant by Hadley in 1731 and of the 

 chronometer by Harrison in 1735. The sextant is an instrument for 

 the measurement of angular distances. As such it replaced the cross- 

 staff and the astrolabe, than which it is far more convenient and ac- 

 curate. The cross-staff required the observer to sight in two direc- 

 tions at once, while the sextant forms two images of the object or 

 objects observed as near together as desired in a small telescope. The 

 astrolabe was suspended and was supposed to be kept plumb by grav- 

 ity, but the movement of the ship rendered accuracy impossible. 

 Three observers were required to manipulate it. The sextant is easily 

 handled by one observer, who, with practise, soon acquires great pro- 

 ficiency and accuracy in the measurement of angles although his posi- 

 tion may be on the unsteady deck of a ship at sea. 



The chronometer is a timepiece like a watch in that it is actuated 

 by a spring and depends upon a balance wheel for the measurement 

 of time. It is however much larger and usually much more accurate 

 than a watch, and it is mounted on gymbals so that it may by its own 

 weight remain face up when its case is tipped. 



In early times mariners used the compass as a rough sundial for the 

 determination of time- Waterclocks and sandclocks were employed 

 for rough purposes of keeping time on board ship, and it is curious to 

 note that hour and half-hour sand glasses were used in the British 

 navy until 1839. When watches were introduced in 1530 they were 

 not accurate enough to supersede even the primitive devices then in 

 use. The practical difficulty arose from their very irregular rates, 

 owing to change in temperature and the motion of the ship. Har- 

 rison's great invention, which made possible the chronometer and 

 greatly improved the watch, was the principle of compensating the 

 balance wheel by the use of two metals with different coefficients of 

 expansion, together with a device by which the chronometer retains 

 its motion while being wound. 



Harrison was eager to try for the longitude prizes with the help of 

 his new invention. He believed that his timepiece, if set and rated 

 carefully before embarking, could be relied upon to keep Greenwich 

 time for a voyage of several months with such accuracy that greatly 

 improved longitude determinations at sea could be effected. In 1735 

 he was allowed to test one of his first watches on a voyage to Lisbon, 

 with a result so satisfactory that he received a grant of £500 to carry 

 out further improvements. The official trial journey to the West 

 Indies was begun in November, 1761, with an improved chronometer; 

 and during the whole voyage of five months the total error unallowed- 

 for was 1 min. 54.5 sec. or the equivalent of 18 geographical miles in 



