128 Mr. R. Phillips on the Electrical Condition 



ence on projecting rods is consequently diminished, a minimum 

 being at last obtained. 



188. When the sun is about to set, the temperature of the air 

 near the ground falls, and consequently the positive orb is low- 

 ered, while the sun's rays still heat the upper regions of the air 

 as at sunrise, and the negative orb is also upborne by the expan- 

 sion previously communicated during the day ; and when the 

 sun is set, the temperature of the air, near the ground rapidly 

 falls. Here are then the conditions for another maximum. 

 Lastly, during night, the temperature of the superior regions of 

 the atmosphere falls, which causes the portion of the negative 

 orb to descend nearer to the positive, and thus produces a dimi- 

 nished electrical effect ; as when the plates of a condenser are 

 closed. 



189. The annual variation is evidently connected with the 

 quantity of rain. Thus the quantity of electricity is greater in 

 winter thaii in summer, because more rain falls in the autumnal 

 than in the summer months. As far as I can see, there are 

 only two conditions under which rain or hail can ever be pro- 

 duced without begetting static electricity ; one is, when the fric- 

 tion to which the drops of water are exposed is so small that the 

 quantity of electricity produced is virtually nothing ; the other is, 

 when the insulation of the different parts of this natural electric 

 machine is so imperfect that the dynamic effect only is obtained. 

 And most probably neither of these two conditions is ever very 

 strictly fulfilled. 



On the Colours of a Jet of Steam. 



190. Professor Forbes some years ago observed, that a jet of 

 steam absorbed the more refrangible portion of white light*. It 

 happened during some experiments, that a blue jet of steam 

 caught my attention, and further experiments soon assured me 

 that it was easy to obtain a jet of almost any colour. 



191. A blowpipe jet was screwed on a T-piece, and the oppo- 

 site opening of the T-piece was supplied with a stopcock, while 

 the third opening of the T-piece communicated, by means of a 

 tube, M'ith the cock of the boiler. The blowpipe jet had an ori- 

 fice about jgyjdths of an inch diameter, and its axis was elevated 

 about 28° above the horizon. The stopcock on the T-piece was 

 furnished with a little contrivance, for preventing the steam that 

 it discharged from interfering with the appearance of the steam 

 discharged by the blowpipe jet ; the use of this stopcock was 

 to blow off the water which condensed in the steam passages. A 

 pressure was maintained in the boiler of about 40 lbs. on the inch. 



192. On fully opening the cock of the boiler, a jet of steam 

 was obtained which appeared blue in nearly every position in 



* Philosophical Magazine, S, 3. vol. xiv. p. 121, 



