ASTRONOMICAL INSTRUMENTS. 135 



When the meridian altitude is required, several observations 

 may be taken, a short time both before and after the meri- 

 dianal passage, with the face of the instrument in one 

 direction, and with it reversed, noting the time at each 

 observation ; and if we have the exact time of the object's 

 transit, its hour angle in time, or its distance from the meri- 

 dian at the moment of each observation, may be deduced. 

 This, with the latitude of the place (approximately known) 

 and the declination of the object, affords data for computing 

 a quantity called the reduction to the meridian, which added 

 to the mean of the observed altitudes, when the object is 

 above the pole, and subtracted when the object is below the 

 pole, will give the meridional altitude of the object, and 

 vice versa, for zenith distances. The nearer the observa- 

 tions are taken to the meridian, the less will the results 

 depend upon an accurate noting or knowledge of the time. 



Price, $500 and upwards. 



Fig. 136. 



The Sun Dial (Fig. 136, as above) is an instrument for 

 showing the hour of the day by means of the sun's shadow. 

 In constructing this instrument, the object is to find, by 

 means of his shadow, the sun's distance at any time from 

 the meridian ; when this distance is known, the hour is also 

 known. Sun dials are usually constructed on a plane sur- 

 face of brass or other material, placed parallel to the horizon, 



