INTRODUCTION. 69 



problem which occurs in nautical astronomy, but is 

 seldom applied. " From the time which the dia- 

 meter of the sun takes to ascend or descend the 

 horizon, to determine the latitude of the ship." 

 The slow motion in ascension of the constellations 

 in high latitudes appears to make this method ap- 

 plicable, at the most, for those regions. 



Lastly, let attention be paid to any instances 

 which may happen to occur of an irregular refrac- 

 tion ; for instance, that of a double horizon, by 

 endeavouring to determine the distance between 

 the false image and the true line of the horizon, 

 wJiich are often visible at the same time, one above 

 the other. These phenomena are usually attended 

 with an unequal temperature of the air at various 

 altitudes, and on the surface of the water, so that 

 it must not be forgotten to examine the temper- 

 ature of the air near the water, and at a greater 

 elevation, on the top-mast for instance, and also 

 that of the water. Similar phenomena, for instance, 

 what is called the Fata Morgana, (French, Mileage,') 

 an unusual elevation of sea coasts that are remote, 

 or even below the horizon, likewise deserve the 

 attention of the navigator, who has to take draw- 

 ings of them, and, if possible, to determine them 

 more precisely by measurements. 



