414 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



TDeckmber, 



ON CONSTRUCTION. 



It is our intention to give occasionally some examples of construc- 

 tion which will be found useful to the student. The annexed engrav- 

 ings show the construction of the Roof over the Polytechnic Institution 

 at Vienna; unfortunately the work (the AUgemehie Bauxeitmig), irom 

 which we take the drawing, contains but a meagre description of its 

 construction, without any reference to the scantling of the timbers ; we 

 can therefore only form out opinion of what they ought to be from the 

 geometrical view before us. It is stated that the roof stands remark- 

 ably firm ; the span is 56 feet, and the rise of the arch 18 ft. 6 in. above 

 the chord bar; the curvilinear ribs or principals are placed 12 feet 

 apart, and are 12 inches in depth, of pieces of timber ia 4 feet lengths 

 which are laid side by side in thicknesses so as to break joints ; we 

 should apportion three thicknesses of two inches each ; upon their 

 ribs are laid the purlins 6 by 5 inches, which carry the rafters 4 inches 

 deep by 2i inches placed 3 feet apart ; upon the rafters is laid the 

 copper covering. The roof is very much stiffened by the braces 6 by 

 3 inches on each side of the ribs, and horizontal ties of the same 

 scantling bolted to the ribs and feet of rafters; and also caulked down 

 to the pole plates, there are two plates G inches square, one lies on 

 the wall, and the other is supported by vertical posts under each 

 rafter; there are also two wall plates each 5 by 5 inches, the wall 

 plates and the foot of the curved ribs ought to be well secured to a 

 cross tie either of timber about 12 by 4 inches, or a wrought iron tie 

 li inch diameter. The tie beam would also answer the purpose of 

 girders to carry the floor — if it be desired to continue the curvilinear 

 form throughout the roof, horizontal ceiling joists 4 by 2 and 12 inches 

 apart might be notched and nailed on to the front edge of the ribs, the 

 laths could then be easily bent to the form of the curve and plastered 



in the usual way; to give the room a pleasing effect, it may be 

 lighted by means of sky-lights in the upper part of the roof, and the 

 centre of the curved ceiling formed into circular sashes and glazed 

 with ground, stained or embossed glass. 



Fig. 1 is a section of one half of the span of the roof drawn to a 

 scale of a quarter of an inch to the foot. 



Fig. 2 is an enlarged view of the purlin secured to the ribs by the 

 iron straps Fig. 3. 



A NEW SAFETY VALNE, 



Sir — It appears to me that the comer in your Journal which was last 

 month occupied by " Funnel," has been filled up but in an indifferent way ; I 

 really can see no object in attempting to arrive at a simple end by means of 

 a very circuitous route ; what possible advantage can the complicated arrange- 

 ment of compensating bars, cylinders and radiating arms have over the beau- 

 tiful, and I may say perfect inveation already in use ? Mr. Funnel should 

 have fixed a cog-wheel at the end of each arm, and a cam to each leg by way 

 of giving his safety-valve a truly eccentric character, and on such he might 

 have grounded its merits, as it is one feels greatly at a loss to ascertain the 

 object of this funny discovery : however, as I hope to spared being levelled 

 to a Caudidus, so must I eschew the ways of that worthy, and by setting all 

 banter apart, must merely venture to suggest that Mr. Funnel would do well 

 to enter into such an explanation of his valve as would tend to estabhsh its 

 superiority, or else by screwing it down to oblivion, would acknowledge 

 tacitly or otherwise, that after all his invention is nothing more than what 

 our mutual friend of the Fasciculi might coll " a mare's nest." 

 Believe me. Sir, to be, with respect, 



Pluc. 



.Voiemier 22. 1841. 



