If the ends of poles are stuck into the earth, and the opposite extremities brought to a common centre and weight--as that of the roof--added, the timbers will sag and a concave section result. That this was one Russian form of roof, the illustration of the Belvedere of the Terem exemplifies (see page 117), where the curve is purposely exaggerated for the purpose of decorative effect. If, instead of being placed loosely in the earth to allow of this set, the poles are thrust down deep into the soil or otherwise made immovable and the upper extremities forcibly brought in towards the centre and fastened there, then when the weight of the roof bends the poles, they will bulge outward in the middle, and when the weight of the roof has been so adjusted as to correct the curve in order to give to the structure the desired greatest possible interior space for domestic accommodation, then the bulbous dome naturally results if the poles be arranged in a circle. The ogival arch is only a section of that. Granted that if the poles cross each other near the tops a more or less concave cone will result--as exemplified in the tepoes of the American Indians--yet if instead of two or three poles, many more have to be brought to the common apex it will be easier not to cross them but bind all firmly to each other--or a central post--then the ogival section must result. If a single pole is bent to form the support of a roof and both its extremities are thrust into the ground, the horseshoe arch is obtained as soon as the weight of the roof acts upon such supports. If, instead of the single pole, two shorter ones are taken and instead of being lashed together to form the pointed arch the upper extremities are brought towards each other and downwards and then lashed, a more rigid bow is obtained, and this is the crude form of the double arch with pendant keystone so common in Moscow; and its use generally is over doorways, etc., where a wide span with great stability is required, and with poles as the only available material this form gives rigidity not obtainable by bending to any other so simple form.