136 Plan for treR'mg a Bridge at London.^ isfe. 



fcnted to the Houfe of Commons, in fiivour of the plan, and the City of London direi^ei 

 George Dance, efq. architeft to the corporation, to examine and verify the local circum- 

 ftances relating to the fame. By the help of fome notes 1 have taken from a feftion made 

 by this archite£t, which I have feen at the Right Honourable Sir Jofeph Banks's, I (ball be 

 enabled to make a few obfervations. 



From ibme experiments made by the Society for the Encouragement of Arts, under die di« 

 reflion of their able fecretary, Mr. Moore, with an appropriate inftrument, for meafuring thlg- 

 reaftion, I underftand that a horfe^ moving at the rate of three miles an hour, can only exert »- 

 force equal to 70 or 80/^. and from the general eftimate of the work of horfes, deduced at 

 page 466 of the fecond vol. tof our Journal, the rea(5lion againft a eommoa horfe, moving at 

 the lame rate, will prove fomewhat lefs than bolb.* and the half of this, namely 30^!^* will turn, 

 out to be the fteady force exerted by a horfe in a poft-chaife, or carriage, which in London 

 is fcldom driven quicker than at the rate of about fix miles and a half an hgur ; though, on the 

 roads near London, they go at the rate of eight, and fometimes near nine miles an hour. I think 

 it likely that there might be fomewhat of temporary effort in the deduftion of Mr. Moore, 

 and that "jolb. may be a good eftimate for the ftout cart-horfes in London ; and, from various 

 circumftances, particularly the over-rated deduflions of feveral refpe£bble authors, we may 

 infer that the animal can double his efforts for a ihort time, fuch as ten minutes, without re- 

 ceiving any injury from the exertion. Thefe fpeculations are not only of general utility, but arc 

 particularly applicable to the fubje<3: before us. If the flope of Mr. Dodd's bridge fliould 

 be fo confiderable as to require additional horfes to furmount it, — or if the length of the inclined 

 plane, required to afford an eafy afcent, Ihould be fuch as to exceed every reafonable computa- 

 tion, with regard to the expences of conftrudlion, and the purchafe of lands, — either of thefe 

 rcfults would afford the moft ferious objeftions to the plan itfelf. Mr.^ Dance's furvey appears 

 to have thefe particulars in view. 



In his drawing, the new bridge is traced 100 feet high in the clear, above high water, and a 

 thicknefs of rather more than ten feet is allowed between the interior curve of the crown oT 

 the arch, and the pavement. Hence the inclination of a line drawn from Monument-yard to 

 the higheft part cf the bridge pavement gives 3,9 inches rife per yard, or very nearly one 

 ninth. If we fuppofe, therefore, an horfe to draw half a ton up this flope, the weight will 

 re-adt with about 120 pounds, and from the length of the line of inclination, it appears that 

 the horfe would have this pull to make for about three minutes. But I muft not overlook 

 that Mr. Dance's arch is taken from the top of high water, whereas, that of the projeftor is 

 ftated, in two places of his pamphlet, to be taken from low-water line ; which makes a differ- 

 ence (as I gather from the two lines in Mr. Dance's feftion) of 22 feet, or more than one- 

 fifth in the whole reaftion, and reduces it to 85 pounds, or very little more than Mr. Moore's 

 horfes at plough did fleadily overcome. If this reafoning be well founded, it will confequently 

 follow, that the fudden afcent of Mr. Dodd's flope will not occafion any inconvenience, or 



* That is to fay, the horfe raifes (or moves againft a re-aftion of) »J hogfheads of water, or 1375'b- (at the 

 rate of or) through ten feet in a minute. But three miles per hour, give th« rate cf 264 feet per minute-} 

 whence 264 : 10 : : 1375 '• ^o* 



render 



