146 TEKM-HOUK AND SPECIAL RECORDS. 



CHAPTER VII. 

 TERM-HOUR AND SPECIAL RECORDS. 



46. The scheme of international co-operation in magnetic work included observations to be made on 

 certain " term days," two a month, and especially during certain term hours, one on each term day. 



The results of the hourly readings taken during the term days the 1st and 15th of each month have 

 already appeared in "Physical Observations," pp. 160-179. The term days all started at Greenwich 

 midnight. On the first of the regular term days, February 1, 1902, the term hour was 0-1 a.m., G.M.T. 

 On the second term day, February 15, 1902, it was 1-2 a.m., G.M.T., and so on, advancing an hour each 

 time for the 24 term days, up to and including January 15, 1903. The scheme included two additional term 

 days, February 1 and 15, 1903, the term hours on which were respectively 0-1 a.m. and 1-2 a.m., G.M.T. 



On the term hours it was intended that the magnetograph drums should be rotated at a higher speed 

 than usual, so as to obtain a very open time scale. 



To change the speed of rotation entailed the attendance of an observer during the term hour, and as 

 many of the term hours including in Europe those on the earlier term days fell during the night, it is, 

 perhaps, not surprising that the number of observatories which took quick runs during all the term hours 

 was very limited. Of the results from the quick-run magnetographs those obtained at Batavia and Pola 

 have already appeared in the official publications of those observatories, where they can be consulted. At 

 Christchurch Observatory quick-run records were" obtained throughout the whole 24 hours of the term 

 day and copies of the curves, together with tabulated results at 20-second or 1-minute intervals, were sent 

 home by Dr. C. COLERIDGE FARR and Mr. H. SKEY, the successive Directors of the Observatory. The 

 labour this entailed must have been very great. At Mauritius quick runs were obtained during 10 term 

 hours, and tracings of these and of the slow-run curves obtained during the other term hours were received 

 from the Director, Mr. C. T. F. CLAXTOJST. 



Quick-run curves were obtained at the National Physical Laboratory on all the term hours. Most of 

 these were taken in the Observatory Department, Kew, but some were taken at Teddington, in the 

 Physics Department. 



As it so happened, the term hours were one and all exceedingly free from disturbances. When a 

 magnetic disturbance of moderate size occurs, a quick-run curve has great advantages, as it admits of much 

 higher precision in the measurements of time. But when magnetic conditions are very quiet, there is a 

 great compensating disadvantage in that the gradient on the quick-run curve becomes so slight that there 

 are no salient features left to catch the eye. There still may remain substantial advantages for tabulating 

 purposes, but, so far as appeals to the eye are concerned, the quick-run curve becomes really inferior to the 

 slow-run. 



Even in 1902 the Kew curves, especially the Vertical-Force ones, were sensibly affected by electric-tram 

 currents. These keep the Vertical-Force magnet in constant oscillation, and though the amplitude is not 

 large it is sufficient to hide from the eye any natural change that is both small and slow. Even at 

 Teddington some of the quick-run curves showed artificial disturbances. 



Taking into account the total absence of any but the most insignificant of natural movements other 

 than the regular diurnal change it was decided not to reproduce the curves obtained at Kew and 

 Teddington. 



An examination of the Mauritius curves showed that, as is usual at most places, the Declination and 

 Vertical-Force curves exhibited less trace of disturbance than the Horizontal-Force curves. As all the 

 curves were very quiet, it appeared unnecessary to go to the expense of reproducing them for all three 

 elements, and the Horizontal Force was selected as the element most worthy of reproduction. The 

 Horizontal-Force curves for all the term hours available appear in Plates I and II. 



