472 



SCIENTIFIC RECREATIONS. 



sensible (apparent) horizon. The true horizon is the circle as on a globe 

 dividing the heavens into two hemi- 

 spheres. The sensible horizon is en- 

 larged according as the eye is elevated 

 above the ground. A man six feet 

 high can see a distance of three miles 

 when standing on a plain. We can 

 always find the distance visible when 

 we know the height at which we stand, 

 or, inversely, we can tell the height of 

 an object if we know the distance. We 

 have only to increase the height one 

 half in feet, and extract the square 

 root for the distance in miles. On 

 giving the distance in miles reverse the 



Operation. Fig - 5oo.-Right ascension. 



For instance, for the man six feet high, as supposed, add three feet, 

 being half his height ; that makes nine feet. The square root (or number 

 multiplied by itself to give nine) is three, which is the number of miles the 

 man can see on a plain. Or, again, suppose we can see a tower on the 

 level, and we know we are twelve miles away from it. The square of twelve 

 is one hundred and forty-four feet, one-third of that is forty-eight feet, which 

 represents the half of the original height added to the whole tower in feet ; 

 so the whole tower is ninety-six feet high. Reversing, as in the former case, 

 we can prove this by taking the tower at ninety-six feet high and trying to 

 find the distance we can see from its summit = 96 + 48 = 144 ; the square 

 root of 144 = 12, the distance required. 



7. The Nadir and the Zenith are the poles of the horizon. The zenith is 



exactly overhead, the nadir exactly 

 under foot. Circles drawn through 

 these points are azimuth circles. 



8. Meridians are circles passing 

 through the poles at right angles to 

 the equinoctial. Every place is sup- 

 posed to have a meridian, but only 

 twenty-four are upon the globe, and 

 they represent the sun's, or the planets', 

 " movements " every hour I 5 being 

 one hour, 360 being twenty-four hours 

 (see fig. 500). One quarter of a degree 

 equals one minute of time. Parallels 

 of latitude are familiar circles parallel 



. *.~ * ***** to the equator. Latitude in astronomy 



Fig. soi.-orbit of planet. j s the distance from the ecliptic at a 



right angle north or south. ,This will be explained as we proceed. 



