Report of the Kew Committee. 



323 



The building in which the absolute observations are made is suffi- 

 ciently remote (about 100 yards) from the main building to be quite 

 unaffected by these sources of magnetic disturbance. 



The photographed magnetic curves representing Declination, Hori- 

 zontal Force, and Vertical Force variations have been secured unin- 

 terruptedly throughout the year, and, as usual, the scale values of all 

 the instruments were determined in January last. 



The following values of the ordinates of the different photographic 

 curves were then found : — 



Declinometer : 1 inch = 0° 22'-04. 1 cm. = 0° 8'7. 



Bifilar, January 5, 1892, for 1 inch SH = 0*0280 foot grain unit. 



„ 1 cm. „ = 0-00050 C.G.S. unit. 

 Balance, January 7, 1892, for 1 inch cV= 0-0287 foot grain unit. 



„ 1 cm. „ = 0-00052 C.G.S. unit. 



In the case of the Vertical Force magnetometer, it was found 

 necessary to readjust the instrument ; at the same time its sensibility 

 was slightly altered, after which the scale value was again deter- 

 mined with the following result : — 



Balance, January 15, 1892, for 1 inch &V = 0'0277 foot grain unit. 



„ 1 cm. „ = 0-00050 C.G.S. unit. 



The distance between the dots of light upon the Horizontal Force 

 cylinder having become too large for satisfactory registration, the 

 trace dot was brought nearer to the zero dot on August 6. 



The principal magnetic disturbances were recorded on the follow- 

 ing dates, viz. :— February 13—14, March 6 and 12, April 26, May 18, 

 July 16 — 17, and August 12. 



The most marked disturbance, however, was that which com- 

 menced on February 13 at 5.34 a.m., and lasted until the afternoon of 

 the 14th. 



The oscillations were of a more extended and violent character 

 than any which have been recorded during the last ten years. 



From the insufficient range of the scale, the magnetometers did 

 not record the complete extent of the vibrations to which the needles 

 were subjected, nor could the entire change of force be secured in the 

 field of the instrument. 



The limits, however, clearly recorded, were 1° 40' of declination, 

 from 0-1755 to 0-1835 of horizontal force, and from 0'4350 to 0-4425 

 units of vertical force expressed in C.G.S. measure in absolute force. 



The following table exhibits the absolute hourly values of Declina- 

 tion (Inclination calculated from the Horizontal and Vertical Forces), 

 the Horizontal Force and Vertical Force having both been corrected 

 for temperature for February 13, 14, and 15, 1892 : — 



2 a 2 



