Determination of the Earth's Horizontal Magnetic Force. 375 



been that of the great feast of the Niceteria in honour of Minerva's 

 contest with Neptune for the protectorate of Athens, and the other, 

 the annual celebration of the Marathon victory. 



As respects the year 447, which is one year earlier than the supposed 

 commencement of the Parthenon, it seems appropriate because at that 

 time Pericles would have been supreme, and it is likely that a temple 

 of such sanctity as the Erechtheum would have called for his earliest 

 attention. It is true that the temple remained long unfinished, but 

 of the causes of this delay we are ignorant. The connection however 

 of the orientation with the feasts above mentioned would have been 

 exactly the same if the date had been 428. 



The apparent discrepance between the orientation date (as respect 

 the day of the month) in the case of the Temples of Jupiter Olympius, 

 and that which is supposed by Mommsen to have been the day of 

 the celebration of the great feast to the supreme god (namely 

 Munychion 19, the tenth month of the year, which in a general way 

 corresponded with April), whereas the orientation dates give for the 

 earlier temple March 30-31, and for the later March 27, is explained 

 by the possibilities of the Metonic cycle, for when the Attic year 

 began as it would in its course on July 11, the 19th Munychion 

 would agree with March 30, or if July 8 with the 27th of March. 



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

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

 Superintendent of the Kew Observatory. Communicated by 

 the Kew Observatory Committee of the Eoyal Society. Be- 

 ceived May 31 — Eead June 15, 1899. 



Contents. 



Sect. 



1 . Introductory. 



2. Temperature coefficients. 



3. Induction coefficient. 



4. Moment of inertia. 



5. Coefficient P. 



6. Tables of mean and extreme values of magnetic constants. 



7. Dimensions of magnets, Table III. 



8 — 14. Discussion of results of Tables I and II. 

 15—18. Relations between different magnetic conslants. 



19 — 24. Probable errors in determinations of horizontal force due to errors in 

 Talues of magnetic constants. 



25 — 28. Criticism of formulae for reducing horizontal force observations, from 

 mathematical standpoint. 

 29. Physical sources of uncertainty, variations of temperature. 



30 -31. „ „ variations of horizontal force and declina- 



tion. 



VOL. LXV. 2 F 



