244 REPOKT — 1866. 



zones selected, as well as at other times, all objects observed that are not to 

 be found on the map nor described in the catalogue, should bo inserted ])ro- 

 Yisionalljs their relative positions having been carefully determined by align- 

 ment or otherwise. 



Drcnuhir/s. — When opportunities occur, drawings should be made of groups, 

 especially such as are indicated of conspicuoiis and easy visibilitj'. (Difficult 

 objects may be sketched when definition is very fine.) Eeaumur, Khirticus 

 IV A« '° IYA«^i with the group near IVA?1, also IVA?25, IV A? ^8, Hal- 

 ley, and Horrox are very suitable for this purpose. 



Measurements. — The most important are for determining points of the 

 first order, and consist of mcasiires taken with the micrometer between the 

 apparent east or west and north or south limbs of the moon and the object; 

 the subject is fully treated with an example of the computation in Ap- 

 pendix il. Report, 1866. The mountain IV A? 29^ ^nd the craters IV A?'', 

 IV A^ ^, and IV A? ^^, appear to be very suitable as points of the first order. 



Measures may also be taken for magnitude : see Report, 1 865, p. 295. 

 The following measures of Halley in a dii'ection perpendicular to the paral- 

 lel, the object being made to run between the wii-es, may serve as an 

 example : — 



1866, June 23, 8" 10" to S'' 25"" G. M. T. 



10-317 9-692 ,r _^ 10-3112-9-7148 .„qq9 



10-307 9-717 ^^'^''' ■■■■ 2 "^^^ 



10-321 9-727 Diouysius 



10-316 9-697 on* o-iA 10-2366-9-7732 



10-295 9-741 



8-0 to 8-10 = ^" "^""7'' '^^^=-2317. 



The magnitude is determined by dividing the measure of the object by 

 that of the standard, thus: Halley =-2982, and Dionysius = *2317, therefore 



2982 

 the magnitude of Ilalley=——— =1-287. 



In reading the micrometer head, the value of the fixed wire from which 

 the positive readings are made, is reckoned equal to ten revolutions, there- 

 fore the negative readings are equal to nine revolutions + the readings of the 

 micrometer head, and as the diftereuce between the two readings will equal 

 twice the measure of the object, it follows that the mean of the positive — 

 the mean of the negative readings — divided by 2, will equal the measure of 

 the object. 



For the reception of the observations above specified copies of Form 

 No. 1 (Report, 1865, p. 287) will be supplied, and when corrections and 

 additions have become sufficiently numerous, duplicate copies of the map 

 wiU be furnished. 



Each observation should be accompanied by references to the following 

 data : — Greenwich mean time ; day elapsed of the Julian Period (D. J. P.) ; 

 the moon's latitiido to the nearest minute, and the moon's nearest distance 

 from Apogee to Perigee with the sign— before or -|- after. At the time of 

 the observations given on p. 242, the moon had passed Apogee 92 hours and 

 wanted 201 hours of Perigee. Objects were therefore west of their mean 

 positions. 



It is desirable that returns should be made at specified intervals not ex- 

 ceeding six lunations. 



