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. 6 h., 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 comparing 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. The commencement of that cycle is placed by ancient chronologers in 1322 A.C. It seems not correct to call 2782 A.C. "the earliest epoch of Egyptian chronology," for we have none of their chronology nearly so old, and in fact no chronology, properly so called, has yet been made out by our Egyptian researches. It is indeed certain that, if the reckoning by heliacal risings of Sirius did not begin in 1322, we must go nearly 1460 years back for its origin; since it must have been adopted when that event preceded only for a short time the annual inundation of the Nile. But, according to some, the year 1322 A.C. fell during the reign of Sesostris, to whom Herodotus ascribes several regulations connected with the rising of the Nile. Certainly, 2782 A.C. is a more remote era than we are hitherto warranted to assume for any astronomical observation. Believe me, dear Mrs. Somerville, Very truly yours, HENRY HALLAM. I refer you to Montucla, if you have any doubt about the Egyptian year being of 365 days without bissextile of any kind. * * * * * I had sent a copy of the "Mechanism of the Heavens" to M. Poisson soon after it was published, and I had received a letter from him dated 30th May, 1832, advising me to complete the work by writing a volume on the form and rotation of the earth and planets. Being again strongly advised to do so while in Paris, I now began the work, and, in consequence, I was led into a correspondence with Mr. Ivory, who had written on the subject, and also with Mr. Francis Baily, on the density and compression of the earth. My work was extensive, for it comprised the analytical attraction of spheroids, the form and rotation of the earth, the tides of the ocean and atmosphere, and small undulations.