ALEXANDRIAN SCIENCE. 47 



the neglect of astronomical data, Julius Caesar, himself possessed of 

 considerable astronomical knowledge, obtained the assistance of a 

 Greek astronomer named Sosigenes to devise some method of adjust- 

 ing the civil year into correspondence with the astronomical year. 

 This was accomplished by every fourth year intercalating an additional 

 day, and thus was introduced the Julian Calendar, which has, with 

 some slight modifications, continued in use to the present day. In it 

 the year is assumed to be 365^ days, and the civil year is made 365 

 days, except in every fourth year, when it is 366 days. The true 

 length of the year is, however, somewhat less than 36 5!, and it was 

 this excess of a few minutes which had accumulated to eleven days in 

 the year A.D. 1752, that rendered necessary the change from the "Old 

 Style" to the " New Style " in Great Britain. 



We have no original scientific discoveries to record of a single 

 Roman. Their labours as regards the knowledge of nature appear to 

 have been limited to collecting the records and observations of others. 

 PLINY THE ELDER (A.D. 23 79) has left the most voluminous writings 

 of this kind, in which he treats of the stars, the heavens, wind, rain, hail, 

 minerals, trees, plants, flowers, all animals, birds, and fishes, together 

 with geographical descriptions of places and histories of arts and 

 sciences. Pliny fell a victim to his scientific curiosity regarding an 

 eruption of Vesuvius, and his name therefore deserves an entry in the 

 list of the " martyrs of science." The connection of the tides with the 

 moon is clearly pointed out by Pliny, who, however, mentions that it 

 had been vaguely known before his time. Pliny states that a sphere 

 of rock crystal was used as a burning-glass for collecting the sun's 

 rays to a focus, and he mentions the power of a globular glass vessel 

 filled with water to produce the same effect, expressing his surprise at 

 finding that the water itself remained quite cold. Seneca also men- 

 tions the magnifying power of a glass of water, and he refers this effect 

 to some power of the water. He notices also the prismatic colours 

 seen in an angular piece of glass. 



A later writer named AULUS GELLIUS (c. A.D. 130) has some in- 

 teresting particulars about optical contrivances, and from some of his 

 statements it would appear that he was acquainted with the fact of 

 concave mirrors producing images which might pass among the igno- 

 rant as miraculous apparitions. 



We may now return to Alexandria, which continued, after Egypt had 

 become a Roman province, to be the seat of almost all the science 

 of the time. It is true that the glory of the place as a school of learning 

 had declined, so that we do not find any names that need be here 

 mentioned from the time of Ctesibius and Hero until we reach the 

 astronomer Ptolemy. 



CLAUDIUS PTOLEM^US (c. A.D. 150), who is usually known by the 



name of PTOLEMY, flourished at Alexandria in the second century 



f the Christian era. His name is not derived from any connection 



