decline of science in england-第14节
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cted； the performance of the six…inch circle will appear fully equal to that of circles of the larger dimension。 The comparison with the two stations； at which a more than usual attention was bestowed； is the more appropriate； because it was essential to the purposes for which the latitudes of the French stations were required； that the observations should always be conducted with the utmost possible regard to accuracy。
〃It would appear； therefore； that in a repeating circle of six inches； the disadvantages of a smaller image enabling a less precise contact or bisection； and of an arch of less radius admitting of a less minute subdivision； may be compensated by the principle of repetition。〃
Captain Sabine has pointed out Maranham and Spitzbergen as places most favourable to the comparison。 Let us take the former of these places； and compare the observations made there with the small repeating instrument of six inches diameter； with those made by the French astronomers at Formentera； with a repeating circle of forty…one centi…metres； or about sixteen inches in diameter； made by Fortin。 It is singular that this instrument was directed； by the French Board of Longitude； to be made expressly for this survey； and the French astronomers paid particular attention to it； from the circumstance of some doubts having been entertained respecting the value of the principle of repetition。
The following series of observations were made with the two instruments。 ＇I have chosen the inferior meridian altitude of Polaris； merely because the number of sets of observations are rather fewer。 The difference between the extremes of the altitude of Polaris； deduced from sets taken above the pole by the same observers； amounts to seven seconds and a half。＇
Latitude deduced from Polaris； with a repeating circle； 16 inches diameter。BASE DU SYSTEME METRIQUE； tom。 iv。 p。 376。 1807。
Number of Latitude Names of Observers。 Observations。 of Formentera。
deg。 min。 sec。 64 38 39 55。3 Biot 100 54。7 Arago 10 56。2 Biot 88 56。9 Biot 120 56。7 Arago 84 54。9 Biot 100 56。5 Arago 102 57。1 Arago 80 54。5 Biot 88 53。3 Arago 90 53。6 Arago 88 53。8 Arago 92 53。7 Arago 42 55。6 Chaix 90 54。1 Chaix 80 53。9 Arago
Mean of 1318 Observations； 38deg。 39min。 54。93sec。
*
Sets of Observations made with a six…inch repeating circle； at Maranham。
Star。 Number of Latitude Observer。 Observations。 deduced。
deg。 min。 sec。 alpha Lyrae 8 2 31 42。4 Capt。 Sabine alpha Lyrae 12 43。8 Ditto alpha Pavonis 10 44。5 Ditto alpha Lyrae 12 44。6 Ditto alpha Cygni 12 42。1 Ditto alpha Gruris 12 42。2 Ditto
Mean latitude deduced from 66 observations 2deg。 31min 43。3sec。
In comparing these results； although the French observations were more than twenty times as numerous as the English； yet the deviations of the individual sets from the mean are greater。 One second and three…tenths is the greatest deviation from the mean of the Maranham observations； whilst the greatest deviation of those of Formentera； is two seconds and two…tenths。 If this mode of comparison should be thought unfair； on account of the greater number of the sets in the French observations； let any six； in succession； of those sets be taken； and compared with the six English sets； and it will be found that in no one instance is the greatest deviation from the mean of the whole of the observations less than in those of Maranham。 It must also be borne in mind； that by the latitude deduced by the mean of 1250 superior culminations of Polaris by the same observers； the latitude of Formentera was found to be 38deg。 39min 57。07sec。； a result differing by 2。14sec。 from the mean of the 1318 inferior culminations given above。 ＇This difference cannot be accounted for by any difference in the tables of refraction； as neither the employment of those of Bradley； of Piazzi； of the French； of Groombridge； of Young； of Ivory； of Bessel； or of Carlini； would make a difference of two…tenths of a second。＇
These facts alone ought to have awakened the attention of Captain Sabine； and of those who examined and officially pronounced on the merits of his observations； for； supposing the skill of the observers equal； it seems a necessary consequence that 〃the performance of the six…inch circle is〃 not merely 〃fully equal to that of circles of larger dimensions；〃 but that it is decidedly SUPERIOR to one of sixteen inches in diameter。
This opinion did indeed gain ground for a time； but； fortunately for astronomy； long after these observations were made； published； and rewarded； Captain Kater； having borrowed the same instrument； discovered that the divisions of its level； which Captain Sabine had considered to be equal to one second each； were； in fact； more nearly equal to eleven seconds； each one being 10。9sec。 This circumstance rendered necessary a recalculation of all the observations made with that instrument： a re…calculation which I am not aware Captain Sabine has ever thought it necessary to publish。 ＇Above two hundred sets of observations with this instrument are given in the work alluded to。 It can never be esteemed satisfactory merely to state the mean results of the corrections arising from this error： for the confidence to be attached to that mean will depend on the nature of the deviations from it。＇
This is the more to be regretted； as it bears upon a point of considerable importance to navigation； and if it should have caused any alteration in his opinion as to the comparative merits of great and small instruments； it might have been expected from a gentleman； who was expressly directed by the Board of Longitude； to try the question with an instrument constructed for that especial purpose。
Finding that this has not been done by the person best qualified for the task； perhaps a few remarks from one who has no pretensions to familiarity with the instrument； may tend towards elucidating this interesting question。
The following table gives the latitudes as corrected for the error of level：
Station。 Star Latitude Latitude Diffe… by Capt。 corrected for rence Sabine error of level。
deg。min。sec。 deg。min。sec。 sec。 Sierra Leone Sirius 8 29 27。9 8 29 34。7 6。8
Ascension Alph。Centuri 7 55 46。7 7 55 40。1 6。6
Bahia Alph。Lyrae 12 59 19。4 12 59 21。4 2。0 Alph。Lyrae 21。2 58 49。8 31。4 Alph。Pavonis 22。4 59 5。1 17。3
Maranham Alph。Lyrae 2 31 42。4 2 31 22 20。4 Alph。Lyrae 43。8 31。8 12。0 Alph。Pavonis 44。5 44 。5 Alph。Lyrae 44。6 42。6 2。0 Alph。Cygni 42。1 39。2 2。9 Alph。Gruris 42。2 27。4 14。8
Trinidad Achernar 10 38 56。1 10 38 58。2 2。1 Alph。Gruris 52。2 50。8 1。4 Achernar 59。3 56。6 2。7
Jamaica Polaris 17 56 8。6 17 56 4。6 4。0 6。6 3。3 3。3
New York Sun 40 42 40。1 40 42 44。6 4。5 Polaris 48。9 38。2 10。7 Sun 41。4 47。2 5。8 Beta Urs。Min。 42。3 58。4 16。1
Hammerfest Sun 70 40 5。3 70 40 7。2 1。9
Spitzbergen Sun 79 49 56。1 79 49 58。6 2。5 Sun 55。9 44。8 11。1 Sun 58。6 52。7 5。9 Sun 59。3 51。6 7。7 Sun 55。8 51。6 4。2 Sun 50 1。5 57。0 4。5
Greenland Sun 74 32 19。9 74 32 32。4 12。4 Sun 17。9 18。7 0。8
Drontheim Sun 63 25 51。3 63 26 6。1 14。8 Alph。Urs。Min。 57。2 49。4 7。8
This presents a very different
