ON DIVIDING ASTRONOMICAL INSTRUMENTS. j /aq 



4111 engine thus appointed would succeed foi- dividing cir- 

 cles? I answer, Yes; but I would not recouimend it; be-Suchanen- 

 cause, beyond a certain extent of radius, it is uot necessary ; "'^'^'^ misrh' fea 

 for tl-ie erroiirs, which would be introduced into the work ing circles,' to 



bv the violence of racking a large wheel, are sufficiently re- '"^^ ^° ^^ •■«- 

 1 1 1 ^1 .• 1 ,. , 1- „ , commended, 



uucecl by the comparative shortness of the radius of such 



instruments as we divide by that method : And, what is still 

 more to the purpose, the dividing engine is four times more 

 expeditious, and bears rough usage better. I cannot quit 

 the subject of dividing straight lines without observing, that 

 I never had my apparatus complete. The standard which Standard mea- 

 I made for Sir George Shuckburg Evelyn in 1796 was done ^"'"'^^^ 

 by a mere uiake-shift contrivance, upon the principle of di- 

 viding by the eye; how I succeeded may be seen in Sir 

 George's papers on Weights and Measures (Phil Trans, for 

 1798). I made a second, some years after, for Professor 

 Pictet of Geneva, which became the subject of comparison 

 with the new measure of France, before the National Insti- 

 tute; and their report, drawn up by Mr. Pietet, has been 

 ably restated and corrected by Dr. Young, as published in 

 the Journals of the Royal Institution. I made a third for 

 the Magistrates of Abertleen. I notice the two latter, prin- 

 cipally to give myself an opportunity of saying, that, if 

 those three scales were to be compared together, notwith- 

 standing they were divided at distant periods of time, and 

 at different seasons of the year, they would be found to 

 agree with each other, as nearly as the different parts of the 

 same scale agree. 



I hope I may here be allowed to allude to an inadverten- Inadvertencj 

 cy, which has been committed in the paper mentioned shuckbuf 

 above; and which Sir George intended to have corrected, Evelyn; 

 had he lived to conclude his useful endeavours to harmonize 

 the discordant weights and measures of this country. The 

 instruments which he has brought into comparison are, his 

 own five feet standard measure and equatorial; General 

 Roy's forty-two inch scale; the standard of Mr. Aubert; 

 and that of the Royal Society. The inadvertency is this; 

 In his equatorial, and the standard of the Royal Society, 

 he has charged the errour of the most erroneous extent, 

 when compared with the mean extent, alike to both divi- 



s.ci.s : 



