LATITUDE AND LONGITUDE. 



95 



varies so little in a year, that for a few days it becomes 

 insensible. 



This is termed finding the latitude by meridian alti- 

 tudes, of which we have the following 



By a meridian altitude of the sun, 

 By a meridian altitude of a planet, 

 By a meridian altitude of the moon, 

 By a meridian altitude below the pole. 

 When we cannot obtain a meridian altitude of the 

 sun and it is necessary to determine the latitude before 

 night enables us to employ the stars, it is then we have 

 recourse to altitudes of the sun taken out of the meri- 

 dian, termed " finding the latitude by double altitudes 

 pf the sun. " 



We can, for example, observe two altitudes of the 

 sun at two different instants of time, having some given 

 interval, say one hour and half; if then, by help of a 

 good watch, we ascertain this interval between the 

 observations, and knowing beside the sun's declination 

 we can readily determine the latitude, and this method 

 is also applicable to any given star, and is termed gene- 

 rally, finding the latitude by double altitudes. 



To demonstrate this method we may imagine in the 



annexed figure that HOR represent the horizon,, H^ 

 RK the meridian, EOQ the equator, 2 the zenith, P 



