460 On Atmospheric Electricity. 



o'clock. Again, in all cases mentioned above, the negative 

 readings were got with a westerly wind, which in Madras is 

 usually very dry, having blown over a large extent of flat dry 

 land. Whenever the wind veered round so as to blow in the 

 least degree from the sea, the potential at once became positive. 

 This has been well marked in all cases, and when the sea- 

 breeze came on early in the day no negative readings were 

 obtained ; but when, as happened on several occasions, the 

 sea-breeze was delayed till late in the afternoon, even then 

 negative readings continued till the wind veered round. An 

 important question is, What is the influence of the clouds of 

 dust that are usually flying on days when we get negative 

 readings ? This is a question which I have not yet been able 

 to answer fully; but certain observations tend to show that 

 there is an intimate connection between the presence of dust 

 and the negative electrification of the air. Many observa- 

 tions were made extending over 10 to 20 minutes: and it 

 was almost invariably found that the negative electrification 

 was strongest during gusts of dust-laden air. At such times 

 the potential would often run up so rapidly that it was im- 

 possible to measure it accurately, while during lulls it would 

 often fall almost to zero. The mere friction of the dry dust 

 against the match and insulated umbrella would not account 

 for this § for the potential began to increase rapidly before 

 the dust-cloud actually reached the place of observation. 

 Again, it was found that higher negative readings were got 

 when the observations were made on the ground than when 

 they were made on the roof of a house ; and, further, that in 

 parts of the town where there was little dust, negative read- 

 ings were rare, and were never very high. Too much stress 

 must not be laid on this last point, for simultaneous observa- 

 tions could not be carried out at two places, as I had only one 

 electrometer. A number of observations have been made in 

 dust with south-easterly and north-easterly winds ; but the 

 air was never found to be negatively electrified. This 

 may be explained readily enough as due simply to the 

 presence of moisture ; for one of the earliest facts im- 

 pressed on an experimenter on static electricity in this 

 part of India is that, however well an electric machine may 

 work, or a Leydenjar hold its charge, while a westerly wind is 

 blowing, the moment the wind veers round to the'east the 

 Leyden jar loses its charge almost at once and the electric 

 machine; ceases to act, be it a Holtz, a Voss, or a Wimshurst, 

 or any other machine warranted to work "in all weathers." 



When the weather becomes sufficiently dry I intend to make 

 some direct experiments on the action of' dust, and on the 

 height to which the negative electrification extends. 



