The construction given in the figure is only the quickest mathematical way of obtaining, on the picture, the sight-magnitudes of _DC_ and _PC_, which are both magnitudes parallel with the picture plane. But if these magnitudes are too great to be thus put on the paper, you have only to obtain the reduction by scale. Thus, if _TS_ be one foot, _TD_ eighty feet, _DC_ forty feet, and _CP_ ninety feet, the distance _QS_ must be made equal to one eightieth of _DC_, or half a foot; and the distance _QP′_, one eightieth of _CP_, or one eightieth of ninety feet; that is to say, nine eighths of a foot, or thirteen and a half inches. The lines _CT_ and _PT_ are thus _practically_ useless, it being only necessary to measure _QS_ and _QP_, on your paper, of the due sight-magnitudes. But the mathematical construction, given in Problem I., is the basis of all succeeding problems, and, if it is once thoroughly understood and practiced (it can only be thoroughly understood by practice), all the other problems will follow easily. Lastly. Observe that any perspective operation whatever may be performed with reduced dimensions of every line employed, so as to bring it conveniently within the limits of your paper. When the required figure is thus constructed on a small scale, you have only to enlarge it accurately in the same proportion in which you reduced the lines of construction, and you will have the figure constructed in perspective on the scale required for use. PROBLEM IX. The drawing of most buildings occurring in ordinary practice will resolve itself into applications of this problem. In general, any house, or block of houses, presents itself under the main conditions assumed here in Fig. 54. There will be an angle or corner somewhere near the spectator, as _AB_; and the level of the eye will usually be above the base of the building, of which, therefore, the horizontal upper lines will slope down to the vanishing-points, and the base lines rise to them. The following practical directions will, however, meet nearly all cases:— [Illustration: Fig. 54.]