PT. i. Astronomy and Geology compared. 29 



in the state of the weather. Pocket chronometers 

 are also of the greatest use to travellers by land in 

 the remote and unknown quarters of the globe, as 

 in the interior of Africa or Australia, or in Central 

 Asia, where a ready and portable means of deter- 

 mining the longitude of places is an invaluable aid 

 to geography. Nevertheless, as the best chronometers 

 are liable to irregularities and cannot always wholly 

 be relied upon, the means of ascertaining the longi- 

 tude by astronomical observation had long been a 

 great desideratum to the navigator. The motion of 

 a ship at sea threw considerable difficulties in the 

 accomplishment of this object, where such great 

 accuracy in the observation is requisite. A reward 

 of 30,000. was long offered in the last century for 

 the attainment of this object ; but although a large 

 portion of this reward was given to Mr. Harrison 

 for his chronometer in 1767, yet, as perfect accuracy 

 has never yet been obtained either by chronometers 

 or, in consequence of the motion of the ship, by 

 lunar observation, the whole 30,000. has never been 

 adjudged. Still, however, the tables constructed by 

 the Board of Longitude, and inserted in the Nautical 

 Almanac, do give the means of determining the 

 longitude with perfect accuracy, subject only to the 



