Comparisons of Magnetic Standards, 1905-14 



243 



Our next check on the po.ssible correction of the Kew Barrow dip circle No. 33 on an 

 international standard i.s aft'ortled by Table 14 E, which gives the results of comparisons of 

 16 new dip circles with the Kew standard between 1902-15. The results for these 16 dip 

 circles are taken directly from the Kew certificates furnished with the instruments and are, 

 accordingly, those obtained by the Kew Observatory itself. It is the custom of the Obser- 

 vatory to give the result of its test for each needle of a dip circle only to the nearest 0'.5; 

 where quantities appear in the table to the nearest O'.l, they have been furnished by 

 the Observatory at our own request, though they are not vouched for to that accuracy. 

 Glancing over the last column, it will be seen that 4 of the instruments were those pur- 

 chased in recent years by the United States Coast and Geodetic Survey, and 12 of them by 

 the Carnegie Institution of Wasliington (Department of Terrestrial Magnetism). 



^.\11 of these dip circles were made by Dover, excepting C18, the maker of which was Casella, the number given it 

 being that of the United States Coast and Geodetic Survey. 



If there be no constant error in the dip-circle method, and at present there is no valid 

 reason for suspecting such a source of error, it would appear that the mean result from so 

 man\' different dip circles should fall not far from the truth. Granting this, it is seen that 

 the general evidence indicates that the Kew standard gives too great values of the incUnation 

 by about I'.O a result in good agreement with that (1'.2) found from Table 14 C and with 

 that ( 1 '.2) fromTable 14 D. The average correction of the Kew inclination standard on I. M. S. 

 is accordingly taken as I'.l. 



.\ssembling the various results as referred to I. M. S. (see p. 273), we obtain: 

 (14) I. M. S.-Kew (Jones magnetometer) = +0'.6 (1908^10). 

 (14a) I. M. S.-Kew (Jones magnetometer) = -0.00008// (1908-10). 

 (144) I. M. S.-Kew (Barrow dip circle No. 33. needles 1. 2)= - I'.l (1902-15). 



NO. 15. MAURITIUS OBSERVATORY, MAURITIUS. 



The comparisons at the Royal Alfred Observatory, situated at Pamplemousses on the 

 island of Mauritius, were obtained in connection with the shore work of the Carnegie, while 

 at Port Louis in August 1911. 



The Observatory magnetometer is of the Kew pattern (Elliott No. 24), as also the dip 

 circle (needles 1, 2, 3, and 4). The latter instrument, together with the needles, owing 

 to the moist, tropical conditions, was found to be in bad condition and was replaced in 1913 



