TELESCOPE. 



Table II — Value of the Parts of the Micrometer's Screw, with four different Tilcfcopcs 



The ufe of thefe tables will be beft underftood from a 

 few real examples. 



1. The fun's diameter was taken by Troughton's micro- 

 meter, applied to the telefcope of 45.75 inclies focal length, 

 on the 27th of May 1815, when his altitude was fo high 

 as to require no correftion tor the difference of the two rc- 

 fraftions of the lower and upper limbs, and was found equal 

 to 43.62 turns of the fcrew : then by Table I. 43 = 

 ■31' io".5, and by Table II. .62 = 26".9, the fiftn of 

 which is 3i'37'''.4, the diameter given in the Nautical 

 Almanac being 31' 37". In this obfervation the thicknefs 

 of the fpider's line was allowed for. 



2. On the 7th of Auguft 1815, the fun's diameter at 

 noon meafured 60.60 turns, when the micrometer was ufed 

 with the telefcope of 63.5 inches focal length ; whence we 

 have 60 in Table I. = 31' 18", and .60 in Table II. = 

 i8".8, making together 31' 36".8, the diameter in the 

 Nautical Almanac for that day being 15' 48".3 X 2 = 

 31' 36".6. When thefe meafures were taken, the telefcope 

 was on an equatorial ftand, and the parallel lines were fo 

 placedj that the fun's body paffed along the fpacc con- 

 tained between them, without any apparent variation of 



altitude, which pofition is neceffary in every obfervation 

 taken with Troughton's micrometer, when the objeft has 

 an apparent motion. 



3. On the 14th of Auguft 1815, the moon's diameter 

 was meafured about 9 P.M. not far from the meridian, 

 when her altitude was about 18', with Troughton's micro- 

 meter, attached to the telefcope of 45.75 inches focus, and 

 was found equal to 41.52 turns of the fcrew ; the horizon- 

 tal fcmi-diameter, according to the Nautical Almanac, be- 

 ing at noon 15' 4", and at midnight 15' o", confcquently at 

 the time 15' l". To the horizontal diameter 30' 2", add the 

 augmentation at 18^ altitude, (from Table IV. of the rr- 

 guJjflte tables,) iiix. 5", and the diameter in altitude will be 

 30' 7". Now from Table I. take the value of 41 turns 

 = 29' 43".5, and from Table II. take the value of .52 = 

 22". 6 ; the fum of which two values will be 30*6". i, 

 which muft be incrcafed by 5", the difference of refrailion 

 at 18', and 18° 30' of altitude; fo that the diameter, whtn 

 the reduftions are all made, is too great by 4" nearly, 

 which error may be in the limar tables, or in the obferva- 

 tion, which was made when the moon's age was only eleven 

 days, and therefore under an unfavourable circumftancf ; 

 O o 2 for 



