74 PHILOSOPHICAL TRANSACTIONS. [anNO 1684. 



to the number of about 6g, alphabetically placed, together with the authors 

 that treat of them ; out of all which the author singles alectryomancy for the 

 subject of this book. 



A further Account of the Bridge at Pont St. Esprit,* with a parallel History 

 of some other Bridges at Rome : in a Letter from Mr. Tancred Robinson, to 

 Dr. Martin Lister. N" l63, p. 712. 



Doubtless you recollect the stately ruins of the modern bridge at Avignon, 

 which has yielded in many places to the extreme rapidity and violence of the 

 Rhone. Its fall in my opinion may be ascribed to three defects : 1 . it was not 

 so multangular as that at St. Esprit : 2. it wanted, in 3 or 4 places, the little 

 arches dividing the feet of the great ones ; and in those parts it has suffered 

 most: 3. the pedestals were not so geometrically and exactly laid, as those of 

 Pont St. Esprit ; their projections were few, and those not gradually contracted ; 

 so that the force of the stream must be the greater on the fabric. 



Now that we are upon this sort of history, I shall venture to carry you from 

 the Rhone to the Tyber, which, though not so swift as the Rhone, yet is sub- 

 ject to greater inundations, as many inscriptions assure us. No river ever had 

 so many bridges built with such magnificence and art, as this ; and though 

 they were more pompous, and rich in rare stones, in sculpture, &c. than that 

 I formerly sent you a draught of from Montpelier, yet they had the like pro- 

 vision for their security and preservation, and their design was much the same; 

 as may be yet seen at Rome, in such old bridges as still remain there. 



Description of a Bridge that may be built 70 Feet long ivithout any Pillar under it. 



N° l63, p. 714. 



A timber bridge may be built 70 feet long, or somewhat more, without any 

 pillar under it, which may be useful in some places where pillars cannot con- 

 veniently be built : it may consist of two such arches of timber, as that which 

 is represented in fig. 1 , pi. 3 ; wherein a c, and b o are beams 28 feet long, and 

 ab 32 feet long. Under the angles are set 2 large braces el and sr. At each 

 end is a wall, on which are laid two beams bh and ad, each 20 feet long; and 

 under these are 2 braces de and rh. There may also be braces at the extremities 

 of the arches, crossing the bridge obliquely. It may be laid with planks, and 

 railed; and beyond the walls causeways yi/ and hn. The length of the bridge 

 cmo is 70 feet; and the height km, IQ feet. 



♦ See N° l60, in this volume. 



