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THEORY AND PRACTICE OF ENGINEERING.

Book II.

their houses is almost flat, and forms a terrace, and this is the case in some parts of Italy .The inhabitants of the north have pitched their roofs very steep, for the purpose of moreeasily throwing oft’ the water; for though it rains more frequently in temperate than inwarm climates, the rains are heavier in the latter, and consequently require less inclinationin the roofs to carry them off; and the temperature is so elevated, that they become dryimmediately the rain is over. Where the rain is more frequent and less heavy, the air ismore humid, the water has more difficulty to run off, and the roof takes longer time todry ; consequently a greater inclination in some degree compensates for this. The incli-nation of the roof depends also upon the material that forms its covering : pan tiles requireless inclination than flat or plain ones, as the water collects in the hollow, and more easily runsaway, and is not so much subject to be driven by the wind under them, as where flat tilesor slate is employed : in very rainy weather, and when fine rain is falling, the under partsof slates and other coverings which have a trifling inclination is almost as wet as the upper,as the water is induced to flow up by capillary attraction: when snow is melting, this isoften found to be the case, and the more so, the more smooth and compact the materialswhich form the covering are; even between two panes of glass, the water will remountbetween the lapping surfaces, and it becomes necessary that a sufficient inclination shouldbe given to counteract this effect.

As the inclination to be given to roofs, then, should increase with the latitude, we mustcommence, after quitting the tropical regions, with something like a fall; we find, for thelatitude of Athens, which is 38° 5', if we subtract 23 J 28', which is that of the Tropics ,there will remain 14° 37' as the inclination to be given to roofs in that city : and uponexamining the roof of the Propyleon, we find its inclination to be 14£°*; that of theParthenon 15$ , and that of the Erechtheum 15°. Rome is situated in 41° 54' north latitude,and deducting that of the Tropics , we have 18° 26' remaining, which affords us the incli-nation indicated by the term tertiarium, as used by Vitruvius : speaking of Tuscantemples, he observes, “ stillicidium tecti absoluti tertiario respondeat,” which makes theentire height of the roof, from the gutter, one-third of the length of the horizontal line ;this system gives precisely an inclination of 18° 26'. The porticoes at Rome do not agreewith this inclination, for they vary between 23° and 25°, and which was owing probablyto the material with which they were covered being different, in order that the pro-portions of the facade should not be made subservient to necessity: when we give to theheight of a roof the fifth part of its span, we have an inclination of 21° 48', which is suffi-cient for flat tiles or slates, and in our latitude we usually make the pitch for pan tiles27° 24', for slates 33° 24', and for plain tiles 25° 24'.

The simplest constructionemployed is undoubtedly thatwhich roofs in an excavationmade out of the solid rock orearth. Trees pitched one againstthe other in a triangular form,and well secured at the base,would serve for rafters, whichwould support either branches,shingles, or cleft pieces laid oneover the other, to keep out theweather; upright boarding orpalisades to close the gables or

ends would then constitute it a *‘8* 20y6 *

rude dwelling.

An improvement upon theabove method would be that ofraising the roof sufficientlyabove the ground to enable theinhabitants to walk erect be-neath it without sinking a holein the earth, and which in manysituations would be damp, andperhaps occasionally filled withwater.

Roofs, Vitruvius informs us,w T ere covered with reeds, andsometimes with clay mixedwith straw; and Pliny observesthat the ancients made use of f ig- 2097.

the shells of tortoises for the same purpose. In Switzerland and in Germany shinglessplit out of oak and fir are, on account of their lightness, preferred to any other covering.

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