There are certain differences in the methods of construction of Palenque and Uxmal which have to be analyzed. The walls at Palenque are composed of compact masses of stones and mortar covered with thick layers of cement. At Uxmal no coatings of cement are used and the walls are faced with well-worked masonry. At Palenque there are great numbers of human figures either formed of thick stucco, or graven upon slabs of stone. At Uxmal there are no human figures, no delineations of caciques, priests, or captive victims. The principle of ornamentation is different. Judging from the condition and appearance of the buildings it is clear that Uxmal must be more modern than Palenque, and this is particularly noticeable upon an examination of the Casa del Gobernador and the Casa de las Monjas. The influences of the tropical climate (Uxmal is half a degree south of the twenty-first parallel) are such as would be expected to act injuriously upon exposed limestone sculptures, for between May and November the rains are heavy and continuous. But the façades of these structures seem to have been very little affected by the weather. In forming opinions respecting the antiquity of ruins, it is perhaps injudicious to give much weight to considerations based upon appearance or state of dilapidation, but in the instance of Uxmal there are circumstances which make it impossible to admit that it belongs to a period more than very few centuries earlier than the arrival of the Spaniards. Thus the preservation of the wooden lintels over the doorways in the Casa de las Monjas must be taken into consideration, when any attempts are made to estimate the age of that building. They have had to bear considerable pressure, for there are heavy masses of concrete and masonry to be supported. With regard to this subject, it is necessary to draw attention to the size and construction of one of the principal rooms which I measured, and it will be understood how the architects were limited or restricted in their actions, and to what extent the employment of wood was found to be requisite. It was a narrow chamber twenty feet four inches long, twelve feet wide, and about twenty-three feet high. The height of the entrance was eight feet, the width six feet seven inches. The interior walls were perpendicular up to nine feet three inches from the ground, and then curved inwards until they approached within one foot of each other. They were capped by broad flat stone slabs. The curvature of the walls was correctly formed, and a considerable gain in space was thereby obtained. It was evident that the Indians were advancing towards a knowledge of the round arch and keystone. This form of rounding the inner roof was not however adopted in all the rooms of the nunnery. Many of them still retained the straight lines of converging walls, as at Palenque. The manner in which the advance in construction had been reached was simple and yet ingenious.