1877.] 



of the Earth's Magnetic Force, 



567 



By bifilar, 6 feet above ground = - 0-000G2 X, 



By intensity imifilar, 38 „ „ „ =— 0-00048 X, 



Mean diurnal range by the bifilar = 0-00166 X, 



Avhere X is the whole horizontal magnetic force. 



Having occupied myself at different times during the last thirty years 

 with questions relating to the magnetic and meteorological variations at 

 different heights in the atmosphere, I have examined my observations of 

 the horizontal magnetic force, as far as they have been reduced, with 

 reference to Mr. Chambers's conclusion. 



In 1847 I made two series of simultaneous observations on the highest 

 point of the Cheviot range of hills, and at Makerstoun in Scotland, 

 about 30 miles distant : the difference of height of the two stations is 

 2440 feet. The first series was made in June with a bifilar at Makere- 

 toun, and an intensity unifilar on Cheviot*. The difference of the daily 

 range of horizontal force at the two stations, as deduced from hourly 

 observations during three days, was certainly less than one twentieth 

 of the whole daily range ; but part at least of this difference was probably 

 due to instrumental causes. 



Eor a second expedition in August of the same year (1847) the same 

 instrument was employed 'on Cheviot ; but a second intensity unifilar was 

 observed at Makerstoun, in addition to the bifilar magnetometer. The 

 observatory on the first expedition was under a tent, on the second it 

 was under ground, a deep cutting having been made for this purpose. 

 The result for the horizontal intensity at Makerstoun showed such 

 differences between the two instruments used there (in the same room), 

 that it was evident the unifilar intensity instrument could not be 

 depended on for small differences in the amount of the variations. 



In the years 1855 to 1858 two bifilar magnetometers of precisely the 

 same construction in every way, devised by me expressly for these com- 

 parisons, were placed, one in the Agustia Malley Observatory, 6200 feet 

 above the sea, the other in the Trevandrum Observatory (lat. 8"^ 31' X.), 

 200 feet above the sea (about 24 miles from the former station). 



The first year's observations of the bifilar in the peak observatory 

 were found to be valueless for this question, as it was discovered that 

 the thermometer, though in the same box, did not show with sufiicient 

 exactness the temperature of the magnet : this was due to the magnet 

 being within a pasteboard box, to protect it more completely from cur- 

 rents of air, while the thermometer bulb was outside this inner box. 

 This source of error was avoided by placing two thermometers with 

 their bulbs on opposite sides of the magnet and within the inner box. 

 The series of hourly observations after the middle of August 1856 is 



^ Some account of the results for the Magnetic Declination obtained from tliis 

 expedition was given to the British Association immediatelj^ afterwards (see Brit. 

 Assoc. Kep. 1847, p. 19). 



