24 PROGRESS OF SCIENCE. 



and was the earliest to lay down the FIRST PARALLEL OF 

 LATITUDE (the 36th), after due observation of the identical 

 duration of daylight (14^ hours) on the longest day in the 

 year, from the Columns of Hercules (Gibraltar) to the Hindu 

 Kush. He attempted to measure the MAGNITUDE OF THE 

 EARTH by means of this parallel and the use of the gnomon ; 

 from Alexandria to Syene (now called Assouan), which lay 

 exactly south, he drew at right angles to his parallel a straight 

 line which would necessarily pass at the poles ; and having 

 ascertained the distance between the two cities (5,000 stadia 

 = 625 miles), and convinced himself (from the gnomon cast- 

 ing no shadow whatever at Syene on the summer solstice) 

 that Syene was exactly under the sun at noon on that day, 

 he measured the angle of the shadow cast by the gnomon at 

 Alexandria on the same day of the year at midday, and 

 finding it to be the fiftieth part of a circle, he easily found the 

 magnitude of the circumference (625 x 5o = 3i,25omiles). li 

 the result was incorrect, and such as a rough attempt could 

 be expected to give, the method was admirable and original. 

 He also studied other geographical problems, such as the 

 direction of mountain ranges, the shape of continents, and 

 the way in which the clouds carry rain. According to some 

 writers, he catalogued 675 fixed stars, and tried to ascertain 

 the distance from the sun and the moon to the earth. 



250 220. Ctesibius invented the pump and other 

 hydraulic instruments ; perfected the CLEPSYDRA (water- 

 clock) by applying toothed-wheels to it. 



250 220. Apollonius of Perga solved the problems of 

 the stations and retrogressions of the planets by means of 

 EPICYCLES and deferents, and next to Hipparchus formed 

 the alliance between Geometry and Astronomy. 



160 145. Hipparchus, the REAL FOUNDER of astro- 

 nomical science, and the scientific genius of antiquity par 

 excellence, I, verified the determination of the OBLIQUITY 

 of the ecliptic of Eratosthenes the earliest attempt at an 

 exact determination of the " constants " of nature ; 2, found 

 the length of the TROPICAL YEAR within 12 seconds, making 

 it 365 days 5 hours 49 minutes ; 3, discovered the eccentricity 

 of the solar orbit ; 4, fixed the position of the line of the 



