98 Dr. C. Chree. Observations on Atmospheric 



Interpretation of Electrograph Record. 



2. The first question is : supposing the apparatus perfect, does 

 the electrograph supply information as to the potential anywhere 

 except at the spot where the stream of water breaks into drops ? To 

 answer this question, one has to consider the influence of the environ- 

 ment, notably the proximity of a lofty building. 



An investigation into this point was made ten years ago by Pro- 

 fessor Exner, of Vienna, who found the equipotential surfaces near 

 a building much deflected from horizontally. His results indicated 

 apparently that for practical purposes the whole building might be 

 regarded as possessing the earth's potential. Whilst it was antici- 

 pated that Exner's conclusions would hold good of Kew Observatory, 

 it appeared prudent as a check to take observations with the portable 

 electrometer, at a series of points in a vertical plane perpendicular 

 to the west wall near the water-dropper. Observations were taken 

 at heights of 3, 6, and 9 feet from the ground, which possesses, it may 

 be explained, a slope away from the building. The base line, starting 

 at the Observatory wall, terminated 57 feet away in a parallel wall 

 11 feet high, belonging to a much lower building. The observations 

 were repeated on several days, but one example will suffice. The 

 potential measurements are in volts, the distances from the Observa- 

 tory wall in feet. 



Table I. 

 Observations on November 6, 1894. 



Mean 

 Distance from wall 3 6 12 18 24' 30 36 42 48 54 potential. 



Potential at height 3 feet .. 4 6 18 38 48 46 34 24 16 6 26 



6 .. 8 18 40 58 88 84 76 68 52 22 56 



9 .. 28 44 76 102 120 120 108 68 36 78 



In forming the means in the last column the results at 3 feet from 

 the Observatory wall were omitted. The readings were uncorrected 

 for variations of potential during the interval occupied by the obser- 

 vations. 



So far as they go, the results are clearly confirmatory of Exner's. 

 They show that the influence of a tall building in pulling down the 

 potential extends to a considerable distance. 



3. The large dependence of the electrograph records on the im- 

 mediate environment of the water jet complicates matters, but this 

 need not prove a serious obstacle if the conditions allow us to regard 

 the problem as one of statical electricity, in which influencing bodies 

 are either stationary or at a distance. On this hypothesis, simulta- 

 neous potentials at any two neighbouring points would stand to one 



