POSITION OF DOUBLE STjWRS. 5^17 



ftinwn that, in the day time, a given angle, delineated^ on a card, Obfervationsand 



J n. 1 , ■ ,■ n 1 r II 1 inferences re- 



ann Ituck up at a convenient diltance, may be tu!l as accurately fpefting the 



mealured by a telefcope furnilhed with this niicromeler, as it changes of re- 

 can be done by any known method, when the card is laid on a jnft^rs ext^reme* 

 table before us ; but this would not anfwer my purpofe. For, ly near each 

 objeds in motion, like the ftars, efpecially when at a diftance °'^®^* 

 from the pole, cannot be meafured with fuch fteadinefs as thofe 

 which are near us, and at reft. The method of illuminating 

 the wires, and other circumftances, will likewife afFe6l the 

 accuracy of the angles that are meafured, efpecially when the 

 diftance of the ftars is very fmall. I fliall therefore have re- 

 courfe to aftionomical obfervations, in order to f«e what the 

 micrometer has adually done. 



January 22, 1802. The pofition of A Orionis was taken. 

 1ft meafure, 52^ 38' fouth preceding; 2d meafure, 54^ 14'. 

 Mean of the two meafures, 53° 26'. Deviation of the mea- 

 fures from the mean, 48'. ^ 



March4, 1S02. II Monocerotis. 1 ft meafure, 28 « 18' fouth 

 following; 2d meafure, 26^ 49'. Mean of the two, 27° 34'. 

 Deviation from the mean, 45'. 



February 9, 1803. a Geminorum. ift meafure, 6** 11' 

 north preceding; ,2d meafure, 4° 48'. Mean of the two, 

 5° 29'. Deviation from the mean, 41'. 



September 6, 1802. n Coronae. ift meafure, 89^ 42' 

 north following ; 2d meafure, 89° 38'. Mean of the two, 

 "89° 40'. Deviation from the mean, 2'. 



When thefe obfervations are confidered, we ftiall not err 

 much if we admit that, in favourable circumftances, and with 

 proper care, the micrometer, by a mean of two meafures, will 

 give the pofition of a double ftar true to nearly one degree; 

 but, as the opportunities of taking very accurate meafures are 

 fcarce, it will be neceflary to have recourfe to forae more dif- 

 cordant obfervations. 



February 18, 1803. ^ Orionis. ift meafure, 72° 58' 

 fouth preceding; 2d meafure, 67*24'. Mean of the two, 

 70° 11'. Deviation from the mean, 2^ 47'. 



But a memorandum to the obfervation fays, that the even- 

 ing was not favourable. We may therefore admit, that in the 

 worft circumftances which can be judged proper for meafur- 

 ing at all, an error in the angle of pofition by two meafures 

 will not amount to three degrees. 



It 



