ERA OF THE GREEK AND ALEXANDRIAN SCHOOLS. 



15 



times, with one exception, that of an annular eclipse of the sun, with reference to which 

 the voice of antiquity is silent. 



The instruments of observation known to the ancient world were of a simple and 

 imperfect kind. The earliest of which we read is a vertical pillar employed to determine 

 the sun's altitude by means of its shadow. It was in general use for this purpose among 

 the Oriental nations. The lofty pyramidal stone which Diodorus Siculus describes as 

 hewn out of the mountains of Armenia by order of Semiramis, and set up in a conspicuous 

 part of the city of Babylon, is conjectured to have been erected with this design. A 

 Chinese record of uncertain date, but undoubted antiquity, refers to an observation on 

 the length of the meridional shadows cast at the times of the summer and winter solstices 

 by means of a vertical pillar. It is scarcely to be doubted that such a simple and obvious 

 way of approximating to the length of the year must have suggested itself at a very 

 remote period. The greatest elongation of the shadow cast by the pillar at noon at mid- 

 winter, its gradual decrease towards the vernal equinox, its greatest declension at mid- 

 summer, and its gradual advance to its maximum elongation again, are points to which 

 we may believe attention was early directed ; and a series of observations, taken with 



precaution, would lead to some results 

 valuable in a rude state of society, and 

 to an agricultural people. The same in- 

 strument was also employed to divide 

 the day into equal parts, by means of 

 lines traced on the pavement, which in- 

 dicated the hour as the shadow of the 

 column fell upon them. There is strong 

 reason to believe that these instruments 

 originated the obelisks of a subsequent 

 age, erected in open squares in the Asian 

 cities, and near the entrances of the 

 Egyptian temples, and that these were 

 intended primarily to answer one or all 

 of the purposes named. It has been ob- 

 jected to this idea that the form of the 

 summit of the obelisk will not allow of 

 the extremity of the shadow being accu- 

 rately defined ; but there is some founda- 

 tion for the surmise that a ball crowned 

 the summit, by which the end desired 

 might be gained. Augustus removed 

 two grand obelisks from Egypt to Rome 

 expressly to be used as gnomons, which 

 conveys the impression that they had previously served that office ; and Manlius placed a 

 ball upon the obelisk erected by him in the Campus Martius, with a view to facilitate the 

 accurate delineation of its shadow. The great importance in astronomy of precisely ascer- 

 taining the hour of any phenomenon occurring, or observation being made, led to various 

 forms of the sundial, and to the construction of instruments for measuring time by night and 

 by day when the sun was obscured. The one in general use was the clepsydra, or water- 

 clock, as its name imports, an hour-glass, water being used prior to sand. The orations 

 of the Greeks and Romans were regulated by this contrivance as to their time of speaking, 

 which was called pleading by the clepsydra. The Tower of the Winds at Athens, other- 

 wise styled the Tower of the Clock, an edifice of the age of Alexander, seems to have 



Obelisk at Heliopolis. 



