14 INTRODUCTION. 



Quadrant of altitude. ...In order to facilitate the performance 

 of several problems, such as finding the altitude of the sun, mea- 

 suring the distance and bearings of places, &c. globes are provid- 

 ed with a pliant narrow plate of brass, divided into ninety degrees, 

 which screws on the brass meridian, and turns every way. This 

 plate is called the Quadrant of Altitude. 



Hour circle. ...This is a small brass circle, fixed under the bra- 

 zen meridian, divided into twenty-four hours, and having an index 

 moveable round the axis of the globe. 



Latitude. ...The latitude of any place is its distance from the 

 equator towards either pole, reckoned in degrees of the general 

 meridian, and is northern or southern according as the place lies to 

 the north or south of the equator. No place can have more than 

 ninety degrees of latitude, because the poles, where the reckoning 

 of the latitude terminates, are at that distance from the equator. 



If circles be supposed drawn parallel to the equator through 

 every degree, or every subdivision of a degree of latitude, these 

 circles are called Parallels of Latitude. 



Longitude. ...The longitude of a place is its distance from the 

 first meridian, in degrees of a circle passing through it parallel to 

 the equator, and is reckoned either east or west. The first meri- 

 dian is an imaginary semicircle drawn through any particular place 

 from pole to pole. The situation of the first meridian, or the place 

 from which the longitude is taken, is arbitrary, and has been fixed 

 differently at different times and in different countries. Formerly 

 the meridian of Ferro, the most westerly of the Canary Islands, 

 Was made, in general, the first meridian ; probably, because the 

 ancient geographers considered it as the most westerly point 

 of the habitable globe ; but at present the English astronomers 

 usually reckon from the meridian of London, or rather that which 

 passes through the observatory at Greenwich ; the French from 

 that of Paris, &c. No place can have more than 180 degrees of 

 longitude, because the circumference of the globe being 360 de- 

 grees, no place can be remote from another above half that dis- 

 tance ; but formerly the French and other foreign geographers, in 

 conformity with an ordonnance of Lewis XIII, reckoned their lon- 

 gitude from Ferro, only to the east, from the 1st to the 360th de- 

 gree, or quite round the globe. The degrees of longitude are not 

 equal, like those of latitude, but diminish in proportion as the me- 

 ridians incline, or their distance contracts as they approach the 

 pole. Thus in sixty degrees of latitude a degree of longitude is 

 but half the length of a degree on the equator. The number of 

 miles contained in a degree of longitude in each parallel of lati- 

 tude is given in the following table. 



