PREFACE.
Extent of the Object, whether in Length or Height: As, to view a Building that is a hundred Foot long, and fifty high; he would have the Diſtance a hundred Foot: To view a Tower ſixty Foot wide, and a hundred and fifty Foot high 3 the Diftance fliould be a hundred and fifty Foot. This Diſtance is not ſtrictly to be underſtood of the Space between the Eye and the Object, but of the Space between that and the Section, the Plan of which our Author calls the Line of the Plan, or Ground-line • for it's often requifite, that the Section be plac'd at ſome Diſtance before the Object, on account of Projectures of Cornices, and other Parts of the Work that advance, as in the Eighth Figure.
The Place of the Eye, with reſpect to its Height above the Ground, ought to be ſuch, as is moſt natural and agreeable to the Object. Thus in Architecture, the Baſements and inferior Parts of a Building are improper to be ſet above the Eye, and their Cornices and Entablatures have but an ill Effect when below it. General Perſpectives indeed require the Sight to be taken at a Bird's View 3 and on other Occaſions the Place of the Eye may be vary'd: but the beſt and mod general Rule is, not to exceed five or ſix Foot Height above the Ground. The Height of the Eye above the Ground, thro' which a Line is drawn, call'd the horizontal Line, is ſet on by the ſame Scale of Proportion, as the Deſign bears to the real Work; and the Point of Sight ſo plac'd therein as may render the Object moſt agreeable. From the Point of Sight, either on one or both ſides in the horizontal Line, you are to ſet, by the ſame Scale, the Diſtance you ſtand from the Section. And by means of theſe Points of Sight and Diſtance, and the Meaſures of the Parts brought on the Lines of the Plan and Elevation of the Section by the ſame Scale; all the Examples of this Volume are reduc'd into Perspective; as is manifeſt on Inſpection of the Figures.
What we would add, by way of Advice, is,
I. That you very carefully obſerve, what the Author underſtands by Breadth, Length, and Height, in his Explanation of the Fifth Plate, before you proceed to practiſe on any Figure 3 otherwiſe you'll certainly miſunderſtand him, ſpecially in the Third Figure.
II. That the Rules of the Tenth and Eleventh Figures be particularly regarded, for avoiding Confuſion in the Plans and Uprights.
III. That from the Diſpofition of the Perſpective-Plans and Uprights, with reſpect to the finiſh'd Pieces in the Twelfth and many following Figures, you would obſerve with what Diſpatch the ſaid Pieces may, without the help of Compaſſes, be delineated by your Drawing-Square; viz. the Perpendiculars from the Perſpective-Plan, and the level Lines from the Perſpective-Upright, or Section.
IV. That you would accuſtom yourſelf in Works that have many Lines, to make the Perſpective-Plans and Uprights for each Part diſtinct, ſo as to prevent all Danger of Confuſion. Thus you may have one Plan and Upright for the Baſement of a Building; and when that is drawn on your finiſh'd Piece, remove them, and place thoſe of the Body of the Houſe; and when that's complete, do ſo by the Attick, &c. always
