1835.] Correction of Index Error. 5 1 9 



suggesting its application to the circles in the British Observatories, as the only 

 fundamental mode of enquiry into their errors, a process which coincides in. 

 almost every particular with that adopted by Mr. Taylor, and which amounts 

 in fact to an aerial re-division of the circle in situ - 



" I do not mention this as in any degree wishing to interfere with Mr. Tay- 

 lor's just claim to independence and priority of invention; but as I consider 

 the method in question to be one of great importance, and likely to supersede 

 every other method of examination, I wish to obviate any misconception which 

 may arise from the appearance in England in any of the journals, &c. of this 

 method, as proposed by myself, without mention made of Mr. Taylor's name— 

 what he had done being then entirely unknown to myself, and my own thoughts 

 having been turned upon the subject in the course of a severe examination to 

 which Mr. Maclean has been subjecting the Cape circle, and respecting which 

 he did me the honor to consult me." 



3. — On a simple mode of Correcting the Index Error in taking observations for 

 latitude. By Lieut. W. P. Jacob, Bombay Engineers. 

 [In a letter to the Secretary As. Soc] 

 I send you the results of a few observations, made lately at Karanja and 

 Bombay, in illustration of a very simple method by which the errors of an 

 altitude instrument, when employed in finding the latitude of a place, may be 

 rendered insensible. It consists in observing pairs of stars which have nearly 

 the same meridian altitude, the one N. and the other S. The errors of both in 

 altitude will then be the same, but with respect to the pole they will be in 

 opposite directions, so that the latitude resulting from the mean of the two will 

 be free, not only from the errors of the instrument, but also from those caused 

 by the uncertainty of refraction. 



In the present instance, the instrument employed was a 10 in. circle, reading 

 10," which bad been subjected to very rough usage, having more than once been 

 bent and reflattened, so that its errors sometimes amount to 40" or more ; each 

 star was observed four times, twice with face to the right, and twice to the left 

 and the observations were afterwards reduced to the meridian, an operation which 

 is very easily and quickly performed by means of the sliding rule : 



At Karanja. At Bombay Light house. 



StaTS obsd. Latitude. Mean of each pair. Stars obsd. Latitude. Mean of each pair. 



fjSCephei, 18 51 07.0 1 , Q ,, ._ r & Persei,. . . . 1S53 23.0 1,„ 



(aPisc.Aust.18 51 31.6/ 18 51 19 - 3 { 7 Eiidani, .. 18 53 41.4/ 13 53 32 - 2 

 r a Persei,.. 18 51 07.6"! 18 51 15.5 r a Geminorum, 18 52 28.3"! 18 53 34.4 



l^Eridani, 18 51 23.4/ \ a Cans. Mins. 18 53 40.5 J . 



18 51 17.4 

 DifF. Lat. of Karanja and") 



Light house by Trig. J. -f-2 16.6 

 Survey, J 



18 53 34.0 Lat. of Light house, 18 53 33. 3. 



Here while the individual observations differ greatly, the several pairs have a 

 close agreement, and would doubtless have been .still nearer, had the altitudes 

 been more nearly equal, for a Persei and 7 Eridani differed more than 2° in alt. 

 By this method with a moderately good instrument, the Latitude might be 

 found iu one night within 1", or much nearer, supposing the catalogues correct; 

 Ma/iadeo, 4th March, 1835. 

 3x2 



