I ©4. J Eitperiments it 'qfierl«tn 



as to the value of the tnlctometer fgale, the crofs -wires in their focus were removed to a 



diftance from each other of five inches nearly on the beam (the former being at 27, and the 



latter at 32 inches),, and then correilly adjufted to this interval on the divided fcaia I mud 



obferve, indeed, that the value of the micrometer fcale was not exadly ten revolutions of tl^c 



fcrew to To inch, as Mr. Troughton defigned ; but this meafure by the fere w*, from 6 trials, 



vjas deficient by 0,0002 inch ; yiz. two ten-thoufandths of an inch were to be added to 



each tenth of an inch meafured bythefnicrometer, and fo in proportion for a lefs quantity ; 



but this correSion is hardly worthinottce. , , 



On the fcale. Inches. 



The interval of the crofs-wires in the microfcope 1 Inch Inch 



and micrometer - -. -, - 3 27 and q2 = c,oooo 



Interval of ditto on anodierpart of the fcale, 7 „a o.,j ^r r ^^^^ 

 ; O; (:'(_ .,;.■ 21 .* ' •^ 20 and 3 1 = 5,0000 



viz. - .. - - -,.:, - - -^ 



Ditto ditto ----- 25 and 30=5,0001 1 



1 therefore fay, this interval was 5 inches corredly, to within lefs than the twenty* 

 thoufandth part of an inch on this fcale. 



Meafurement of the cube, viz. of the fide i (fee fig. 5.) 



Inches. Mean. 



From ir ito^=:5 — ,0114 therefore = 4,9886'] Inches. 



axo c:=s-,oiiS = 4^9885 » 8882. 



,^to ^'zzs— ,0105 = 4,989s f ^'^ 



^to ^=5 — ,0113 = 4)9^87 



The fide 2. 



1 



From a to ^=5 — ,0106 = 4,98941 



a to .=5-,oo98 = 4,9902 ( ^4,98955 



c to (i=5 — ,0102 — — = 4,9898 r ^'' ^•3-* 



<sf toi— 5— ,0112 = 4,9888.1 



Height of the cube, from fide i to fide 2. 



From ff to (7=5 — ,0110 = 4,98901 



h to 3=5-,oio5 = 4,989s ( =4,98925 + 



c to f=5— ,0107 = 459893 ( ^'^ ^ ^^ 



diod^^ — ,0108 == 4,9892 J 



• One revolution of the fcrew of • the microhieler was ,Jo inch. 

 Each grand divifion, of which' there were ten . . ,„"„„ inch, 

 Thefe again fubdivided into five each, became . . 5„-o„ inch. 

 And half a divifion, which is very vifible, is . . ' . ^^^^„ inch. 



. ■+ It cannot efcape notice, that all thefe meafures were fomcthing lefs than 5 inches, the quantity propofed ; 

 it arofe from this, Mr. Troughton informs me, he was more folicitous to obtain a true figure, than the exaiSt 

 file ; neither of whicn, however, were very important, as both were to be proved by the mode I have adopted. 

 What was important was to have the fides true, planes, and thefe were examined, as I am informedj by the 

 reflefted image of the moon feen through alarge telcfcope, the focus of which would be altered, if the fur- 

 face were either hollow or convtx.. ' 



§ 13. Now 



