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appears to me a duty, as a member of a literary academy, 

 overcomes my reludance, and calls upon me to contribute fome- 

 thing, though I may be afhamed of the infignificancc of my 

 contribution. 



I WAS prefent, in 1773, at a fet of experiments that were 

 tried in London to determine the relative advantage of high 

 and low wheels for carriages. Difputes had arifen upon this 

 fubjed between mechanics of no fmall eminence, and to deter- 

 mine them an apparatus was provided, confifting of a very long 

 and fmooth table, upon which the carriages to be compared were 

 to be drawn by a firing and a defcending weight. The car- 

 riages were conftruded by fome of the beft workmen in London ; 

 the firings were made of plaited filk of fmall diameters, pafTing 

 over a pulley nicely turned, and mounted in fuch a manner as 

 to have fcarcely any fridion. The experiments, however, were 

 undecifive, each party claiming their evidence in favour of their 

 own opinion ; very little difference was perceptible between the 

 carriages when they ran upon the fmooth table ; and when 

 they were drawn over obflacles, fometimes the high and at other 

 times the low wheels had the advantage, according to the dif- 

 ferent heights and fhapes of the obftacles. It appears upon a 

 firfl view that the force which drew thefc carriages was em- 

 ployed only in overcoming the fridion of the axle-tree, or in 

 lifting the weight over the obftacle. But I fufpeded at the time, 

 and have been fince convinced, that an obftrudion of another 

 fort exifted, which was more confiderable than either of thofe 

 which I have mentioned, and which has not to my knowledge 

 been taken notice of by any writer upon mechanics. 



The 



