 The shape I have adopted is that of an ellipse , formed by bisecting , on each side , the horizontal ordinates of a circle of 32 feet diameter , down to its center , below which the sides are to be made vertical , if no arch is necessary lower down ; otherwise , the ellipse is to continue down to the bottom and to be shaped all round with bricks , stones , or iron , as may appear most eligible . This shape leaves ample room for the cars , and , by contracting the roof , is more secure than a semicircle of 16 feet diameter . 
