The section of a gallery given in the sketch to the right below shows the roof to be constructed on the principle of superimposed horizontal layers, projecting one beyond the other, and each furnished with a sort of heel, which appears to be undercut, so as to give the appearance of a beam from within.  Granite alone would be adequate to the dimensions here given to the key stone, as the thickness of the layers can hardly be considered to be less than a foot.  In taking this as the basis of our calculation for the dimensions of the whole construction, the width of the chamber would be about 25 feet but, judging from the number of urns it contains—­and there is no reason to suppose that these urns were larger than usual—­it would seem to be no more than about 8 or 10 feet.

The construction of the vaults resembles those in the galleries of some etruscan tumuli, for instance the Regulini Galeassi tomb at Cervetri (lately discovered) and also that of the chamber and passages of the pyramid of Cheops and of the treasury of Atreus at Mycenae.

The upper cone displays not only analogies with the monuments mentioned in the note, but also with Etruscan tumuli, such as the Cocumella tomb at Vulci, and the Regulini Galeassi tomb_[Footnote 1:  See FERSGUSON, Handbook of Architecture, I, 291.]. The whole scheme is one of the most magnificent in the history of Architecture.

It would be difficult to decide as to whether any monument he had seen suggested this idea to Leonardo, but it is worth while to enquire, if any monument, or group of monuments of an earlier date may be supposed to have done so._[Footnote 2:  There are, in Algiers, two Monuments, commonly called “Le Madracen” and “Le tombeau de la Chretienne,” which somewhat resemble Leonardo’s design.  They are known to have served as the Mausolea of the Kings of Mauritania.  Pomponius Mela, the geographer of the time of the Emperor Claudius, describes them as having been “Monumentum commune regiae gentis.” See Le Madracen, Rapport fait par M. le Grand Rabbin AB.  CAHEN, Constantine 1873—­Memoire sur les fouilles executees au Madras’en .. par le Colonel BRUNON, Constantine l873.—­Deux Mausolees Africains, le Madracen et le tombeau de la Chretienne par M. J. DE LAURIERE, Tours l874.—­Le tombeau de la Chretienne, Mausolee des rois Mauritaniens par M. BERBRUGGER, Alger 1867.—­I am indebted to M. LE BLANC, of the Institut, and M. LUD, LALANNE, Bibliothecaire of the Institut for having first pointed out to me the resemblance between these monuments; while M. ANT.  HERON DE VlLLEFOSSE of the Louvre was kind enough to place the abovementioned rare works at my disposal.  Leonardo’s observations on the coast of Africa are given later in this work.  The Herodium near Bethlehem in Palestine (Jebel el Fureidis, the Frank Mountain) was, according to the latest researches, constructed on a very similar plan.  See Der Frankenberg, von Baurath C. SCHICK in Jerusalem, Zeitschrift des Deutschen Palastina-Vereins, Leipzag 1880, Vol.  III, pages 88-99 and Plates IV and V. J. P. R.]