To secure this, an edifice should be designed upon well-known geometrical principles: such as science has demonstrated to be necessary and sufficient for firmness and durability. It is well, also, that it have the appearance of stability as well as the reality; for should it seem tottering and unsafe, the sensation of fear, rather than those of admiration and pleasure, will be excited in the beholder. To secure certainty and accuracy in the application of those principles, a knowledge of the strength and other properties of the materials used is indispensable; and in order that the whole design be so made as to be capable of execution, a practical knowledge of the requisite mechanical operations is quite important.