50 HISTORY OF SCIENCE. 



mined, was known to the Greeks from a very early period, but the 

 Alexandrian astronomers seemed to have superseded it by armillary 

 spheres of various kinds. These consisted of metallic circles or rings 

 fixed in a position corresponding with the position of the meridian, 

 the ecliptic, the equator, etc. Thus, for observation of the equinox 

 they used two large circles attached to each other, the one fixed in 

 the plane of the meridian and the other in the plane of the equator : 

 at the moment of the equinox the latter circle was presented edgeways 

 to the sun, so that the shadow of the outer portion fell upon the inner 

 edge. For the solstice one circle fixed in the plane of the meridian 

 had another concentric circle in the same plane revolving within it, 

 and carrying two pins projecting one at each extremity of a diameter, 

 and perpendicular to the plane of the circle. To observe the sun's 

 altitude the inner circle was turned until the shadow of one pin com- 

 pletely covered the other, when the graduations on the rims of the 

 circles gave the altitude of the sun's centre. Ptolemy improved on 

 the solsticial circle by substituting a quadrant. He also invented a 

 simple method of observing the altitudes of stars by means of a pair 

 of jointed rulers, one of which being maintained in the vertical posi- 

 tion while the other was pointed at the star, the angle between these 

 directions was inferred by the distance intercepted on a third gra- 

 duated ruler between the other two. Considering the imperfect ap- 

 pliances at their disposal for observations and measurements, it is a 

 matter for wonder that the ancient astronomers should have been able 

 to accomplish so much. The want of an instrument for accurately 

 measuring time was a great deficiency, for the chronometer is the very 

 soul of practical astronomy. Some of the ancients endeavoured to 

 use the clepsydra, or water-clock; but this instrument was so liable to 

 give erroneous indications that Ptolemy rejects it altogether. The 

 clepsydra was simply an arrangement for showing the quantity of 

 water which escaped from a small orifice in a large vessel, but the rate 

 of flow in such an arrangement is liable to irregularities which cannot 

 be provided for. In default of clocks the ancient astronomers had 

 resort to a rather ingenious expedient when it was necessary to ascer- 

 tain the hour at which an astronomical occurrence took place. Sup- 

 pose, for example, that an eclipse of the moon was being observed, 

 the times of its commencement and termination were found by the 

 observer taking the height at those instants of some well-known fixed 

 star. From this observation, the latitude of the place, and the known 

 position of the sun, it was not difficult to deduce the time. The same 

 method was used by astronomers down to the period when the clock 

 was invented. 



Besides the great work on astronomy which Ptolemy produced, he 

 compiled a remarkable and voluminous treatise~75rr geography. The 

 materials for this were collected from a multitude of authors who had 

 given descriptions of countries and travels, and these materials were 



