200 Mary Somerville. 



FROM HEXRY HALLAM, ESQ., TO MRS. SOMERVILLE. 



WIMPOLE STREET, March, 12th, 1835. 



MY DEAR MADAM, 



As you will probably soon be called upon for 

 another edition of j r our excellent work on the " Connexion 

 of the Physical Sciences," I think you will excuse the 

 liberty I take in mentioning to you one passage which 

 seems to have escaped your attention in so arduous a 

 labour. It is in page 104, where you have this sen- 

 tence : 



"The Egyptians estimated the year at 365 d. 6h., 

 by which they lost one year in every 14,601, their Sothiac 

 period. They determined the length of their year by 

 the heliacal rising of Sirius, 2782 years before the Chris- 

 tian era, which is the earliest epoch of Egyptian chron- 

 ology." 



The Egyptian civil year was of 365 days only, as we 

 find in Herodotus, and I apprehend there is no dispute 

 about it. The Sothiac period, or that cycle in which the 

 heliacal rising of Sirius passed the whole civil year, and 

 took place again on the same day, was of 1461 years, not 

 14,601. If they had adopted a year of 365 d. 6h., 

 this period would have been more than three times 

 14,601 ; the excess of the sidereal year above that being 

 only 9' 9", which will not amount to a day in less than 

 about 125 years. 



I do not see how the heliacal rising of Sirius in any one 

 year could help them to determine its length. By com- 

 paring two successive years they could of course have got 

 at a sidereal year ; but this is what they did not do ; 

 hence the irregularity which produced the canicular cycle. 



