54.' TRANSIT ADJUSTMENT. 



The ufual me- TO make the line of collimation move in the plane pf the 

 thod of adjufting j^p^j jl^j^ we are defired to obferve the tranfits of circura- 



atranfit inftru- ' . ^ , , 



mentbyacir* polar flars, and U the intervals between the times of their 



cumpolar ftar. (ranfits are equal, the traiifit inftrument moves in the place of 



tl>e meridian : for the axis and line of collimation being pre- 



vioufly adjufted, it mutt pafs through the zeniih j and if it 



divides the circle defcribed by any circumpolar ftar, into two 



equal parts, it muft pafs through the pole. 



— requires the But here a difficulty arifes which is a probable alteration, or 



clock to keep a want of uniformity in the rates of the clock or watch for 



time for at leaft ■' , , - , 



ad hours. 'o «o"g a period as twenty.rour hours, or durmg that portion 



of time which the obferver may require to repeat his obferva- 

 tions, fo as to be fatisfied. 

 Method inde- A method independant of the rate of a clock or watch for 

 pendant of this fg jQ^g a time, and alfo entirely of any other previous obfer- 

 Yious^ right vations of right afcenfion, is a defideratura to pra6iical aftro- 

 afcenfion, &c, nomers, and alfa to thofe wlio occaiionally amufe themfeives 

 by obferving time, and the rates ©f their chronometers, in 

 their prefent improved ftate ; but who may be unacquainted 

 with afironomical equations, of precefflon, nutation, &c. 

 Hulc, obferve Rule, Obferve the difference of tranfits of any two circum- 

 the tranfits of polar ftars, that are fituated nearly in the fame azimuth, or 

 two different . , . , , , , , ■ i i . i 



itars, one above vertical Circle, the one above and the other below the pole : 



and the other and whofe difference of right afcenfion is nearly 180"; 

 below the pole; , . 



which differ only (namely J 



a fhort time, for Obferve, The tranfit of a caffiop. above the pole, and 

 fiw'^inutes"' at* ""'"^^'^^^'y after it the tranfit of f urfae majoris below the 

 any time after- pole, whofe difference of tranfit is not more than 15 minutes, 

 wards repeat ^^^ jr^j. f^ {^^^j.^ ^ ^jj^^g jj^g j,|q^.|^ ^j. ^^t^h may be fafely de- 

 the oblervation _ , ^ -' 



upon the ftars pended on. Then invert the operation, and obferve the tranfit 

 when their ^f^ caffiop. below, and s urfas above the pole. If their difference 

 Jituations as to _ . , „ • i i i ,- • , r ■ n 



the pole are ot Iranuts IS the lame in both obfervations, the tranfit initru- 



revcrfed. If the nient is accurately in the meridian; if not the error maybe 

 time be the fami ^^'"''^'^^^ by altering the pofition of the inftrument tii! their 

 the tranfit is difference of tranfit is the fame in both obfervations. 

 not^it^muVbe Should the error be great it may be corrected nearly by any 

 altered, &c. of the theorems now in ufe; {vide Wales on Time-Keepers) 

 or half the difference may be fubftituted for the error, and by 

 repeated approximation the tranfit inftrument may be accu- 

 rately adjufted. 



The 



