188 VEGETABLE SUBSTANCES. 



In many places there are still roofs of this con- 

 struction, though few of the dimensions of that at 

 Edinburgh, and none equal to Westminster-hall. 

 Wherever these roofs are found, they are built 

 not only of the best sorts of timber, but of timber 

 very superior of its kind. In Scotland, they are 

 generally of oak; and the oak of that country, be- 

 fore it was cut' down almost to extermination in the 

 lowland countries, was very compact and durable. 

 In England, several are of chesnut, and some, we 

 believe, of walnut tree ; both of which are, in hard- 

 ness, durability, and beauty, well fitted for the pur- 

 pose. 



The Riding-house at Moscow has the largest roof 

 of a modern construction, and probably occupies 

 the largest area that ever was covered by a single 

 roof, unsupported by cross walls, or pillars. The 

 length is one thousand nine hundred and twenty 

 feet, and the span of the roof two hundred and 

 thirty-five; the floor has the vast extent of more 

 than ten English acres. This enormous roof 

 was supported by a ponderous rib of timber, of 

 a curved form, and made up of three smaller 

 ones, strongly bolted and strapped with iron. The 

 tie-beam by which this immense rib was pre- 

 vented from separating at the extremities, consisted 

 of seven lengths of the largest timber, and the 

 whole truss thus formed was stiffened by cross-pieces 

 and strutts. The weight was so great, however, that 

 the tie-beam swagged, or bent downwards, until the 

 whole truss had the appearance of two curves joined 

 at their extremities. This roof was constructed 

 under the auspices of the Emperor Paul, then Grand 

 Duke, in the year 1790. 



The span or width of the roof of the Theatre at 

 Oxford, is seventy feet, and the length eighty; but 



