Medfurentent of a Bafe on Hounilow-Heath * i 440 
■method, between the point of a micrometer fcrew, fupplying 
the place of the firft cock, and the fecond) the ivory interfection 
Was at firft neceffarily carried beyond the diamond line, foas tb 
make the intermediate fpace lefs than it thould be, until by the 
gradual grinding down of the moveable bell-metal button, it 
was enlarged to twenty feet, as then fhewn by the accurate 
^incidence 6f the interfeQion with the diamond line. 
It was by thefe difiances in the great plank, prolonged to 
twenty- five feet, that the new length -of the Heel chain was 
now fettled, fo as to obtain the full one jhtmdred feet at four 
meafurements. At this time too, brafs points were introduced, 
into the chain at every twenty-five feet, whereby its extent may- 
be compared on any future occafiom; but the temperature had 
now fallen to 66°f » 
Difpoftion of the Stands for the double ?neajurement with the 
Chain and Glafs Rods ; defcription of the apparatus then applied 
to the ends of the Chain ; and ultimate continuation of the 
meafurement with the Glafs Rods alone „ Tab. XVII. and 
XIX. 
From the various cifcumftances already mentioned, in the 
courfe of this tedious, yet necefiary recital, it had been for a 
confiderable fpace of time forefeen, that the refult given by 
the meafurement with the deal rods mult be entirely reje&ed, 
generally in excefs, from handling the inftruments. One degree of alteration, 
producing a difference of about -psWth part of an inch -in the twenty feet, was 
very eafdy and accurately allowed for by fucn a micrometer as this, which fhewed 
the coincidence of the ivory interfedtion with the diamond line to be more or lefs 
perfeft, when the head of the fcrew was moved two divifions, that is to fay, 
or fivsth part of an inch. 
N n 11 % an! 
