462 Proceedings of the Royal Irish Academy. 



com m and of tlie Khalif Almamun, son of tlie illustrious Haroun. This 

 probably came out too late for llessahalali to avail himself of it "when 

 "vrriting the work which we ai'e now coneemed, if indeed he were still 

 alive at that time. 



The zodiacal sign, Pisces, is refen'ed to in Chapter 14 (by 

 Duodecimalis), and in Chapter 32 (by Messahalah), and elsewhere, six 

 times altogether, and is invariably wiitten in the singular, Piscis. 

 It occui's thus once (and also as Pisces) in Gerard's ti'anslation, 

 Latin A. 



In Chapter 8 Duodecimalis gives us a very interesting discussion 

 on geological subaerial denudation. It reminds us of what may be 

 found in Avicenna on the subject. The idea that subaerial denudation 

 has been the great carver of the sui-face features of the Earth is com- 

 paratively recent amongst geologists. Hutton was the first to bring 

 it strongly and prominently forward in his Theory of the Earthy in 

 1785 and 1795 ; though he was not the originator of it. But in this 

 MS. we iind that Duodecimalis had a fail' idea of it above 400 years 

 before. 



In Chapter 11 we have from Duodecimalis an account of the 

 Tides very similar to what we have in Strabo and in Pliny. (Pliny's 

 Natural History was greatly used in the ^iliddle Ages). This account 

 of the tides, though, of course, very imperfect, is sui'prisingly good for 

 the time. It has been known from the time of Posidonius before the 

 Christian Era that the tides are connected with the lloon, and 

 probably produced by her in some way. Kepler, in 1609, threw out 

 the idea, apparently original with himself, that they are caused by a 

 magnet-like attraction of the Moon. Lalande points out that he was 

 anticipated ia this by the Jesuits of the University of Coimbra in their 

 Commentary on Aristotle's Meteor cloy ica, published in 1596, thirteen 

 years before. But we find that they were themselves anticipated, on 

 this matter, in otu' Irish iis., which was written about 200 years 

 before their Commentary. 



In Chapter 34 Duodecimalis makes the ratio of the cii'cumference 

 of a circle to its diameter to be as 3 to 1. If he were a Jew, like 

 Messahalah and many other prominent " Arabian " asti'onomers, he 

 would probably feel himself bound to go by 1 Kings vii. 23 ; though 

 this is not to be taken literally. But it was known long before among 

 the Ai-abians, as by Alfergani, in. the time of llessahalah, that the 

 ratio is, as was shown by Ai'chimedes, veiy nearly as 34- to 1 . 



It is observable that there are fewer references to the Creator, and 



