122 Messrs. Thorjfls and Rucker on some 



Hampshire, U.S. Mr. Burt says : — " I saw a ball of fire as 

 large as a man's head in front of me, not three feet off. It 

 exploded with a tremendous noise. My left leg seemed to 

 be completely paralysed, and I fell to the floor. Three of my 

 printers were in the room at the time, two sitting at the table 

 near me, and one standing up a little further off. The latter 

 had the skin of one hand torn, another was hit in the back, 

 and the third escaped without injury." A tree-like mark was 

 found on Mr. Burt's back. 



This seems also to be a case of glow-discharge passing into 

 an ordinary disruptive discharge. 



We trust the readers of the Philosophical Magazine will 

 excuse the elementary nature of the foregoing details ; but it 

 seems to be necessary that Science should sometimes lift up 

 her voice to point out how so many of the effects of lightning 

 which every year are described in the newspapers as unusual, 

 eccentric, &c, are ordinary events to be explained on well- 

 known principles. 



Highgate, N., 

 July 6, 1888. 



XV. Note on some Additions to the Kew Magnetometer. By 

 T. E. Thorpe, Ph.D., F.R.S., and A. W. Rucker, 



i 



M.A., F.R.S* 



N making field observations with the Kew magnetometer it 



is important that the mirror by which the image of the 

 sun is formed should either be in perfect adjustment, or that 

 the errors should be known and allowed for. 



The axis about which it rotates is made horizontal by means 

 of a riding level. The plane of the mirror is made parallel 

 to the axis by adjusting it until the image of the cross wires 

 formed by reflexion in the mirror does not alter its position 

 when the mirror is inverted in its bearings. The axis of the 

 mirror is made perpendicular to the optic axis of the telescope 

 by making the cross wires and their image coincident. In the 

 course of the Magnetic Survey of Great Britain and Ireland, 

 on which we have been for some tiir.e engaged, we have found 

 such great difficulty in making these adjustments in the field, 

 that we have for a long time practically abandoned this 

 method, and adjusted the mirror from time to time, either 

 indoors by means of a plumb-line, or by observations on some 

 convenient object external to the instrument in the field. We 

 have always, when possible, made four sets of sun observa- 

 tions, reversing the mirror in its bearings, and taking " front " 



* Communicated by the Physical Society : read June 0, 1888. 



