306 



Collimator Magnets. Thermal Expansion. 



" Collimator Magnets and the Determination of the Earth's Hori- 

 zontal Magnetic Force." By C. Chree, Sc.D., LL.D., F.E.S., 

 Snperintenclent of the Kew Observatory. Communicated by 

 the Kew Observatory Committee of the Eoyal Society. 

 Eeceived May 31, — Bead June 15, 1899. 



(Abstract.) 



During the last forty years, there have been examined at Kew 

 Observatory upwards of 100 collimator magnets used in observing the 

 horizontal force and declination. 



The " constants " of these magnets — temperature and induction 

 coefficients, and moment of inertia — have been determined at the 

 Observatory, and the tables based on these determinations have 

 served to reduce magnetic observations at a large number of the 

 leading magnetic observatories. 



The present paper deals with the data recorded in the Observatory 

 books for the constants specified above, and with other quantities — 

 such as the " permanent " magnetic moment — which are deducible from 

 the records. It determines the mean values of the several quantities 

 for the instruments of the leading English makers, and investigates 

 whether relations do or do not exist between them. It then deduces 

 from the records the probable errors in the values of the several 

 quantities, proceeding on the hypothesis that the methods of deter- 

 mining them are correct. It next examines, from a mathematical 

 standpoint, the accuracy of the formulae employed in reducing hori- 

 zontal force observations, and, from a physical standpoint, the possibility 

 of differences between the quantities determined at the Observatory 

 and the quantities actually concerned in horizontal force observations. 



The various sources of uncertainty are dealt with, and an attempt 

 is made to ascertain to what extent they may affect the values found 

 for the horizontal force. 



The results of the paper are of too technical a character to admit 

 of their being summarized briefly in an intelligible way. 



" The Thermal Expansion of Pure Mckel and Cobalt/' By A. E. 

 Tutton. B.Sc. Communicated by Prof. Tilden, D.Sc, F.B.S. 

 Eeceived April 18, — Eead May 5, 1899. 



The following are the numerical experimental data of the eighteen 

 individual determinations of the coefficients of expansion of pure nickel 

 and cobalt, referred to in the abstract previously published (p. 161, 

 supra). Full explanations of the signs employed in the tables will be 

 found in the memoir " On the Thermal Expansion of certain Sul- 

 phates."" 55 " 



* 'Phil. Trans.,' A, vol. 192, p. 455. 



