118 Mr Macvicar on a remarkable electrical Cloud. 



ought to be bright, was the most active, and- the cloud which 

 surmounted it was drawn down in a similar manner, though 

 not so remarkably. These clouds moved towards the east, and 

 when they passed through the region where the rainbow was 

 formed, they had the effect of insulating the colours much 

 more perfectly than when they were developed in an ordinary 

 dark nimbus. When these clouds passed away, the upper 

 stratum of aqueous matter presented that mottled appearance 

 (like tin-plate mottled by a very weak acid,) so often observed 

 when a cloud is giving off lightning, or insulated in a different 

 electric state from the ground beneath, or possessed of that 

 quantity which constitutes the natural equilibrium of a cloud. 

 I mention this phenomenon, not as if it were the index of an 

 unusual state of a cloud, but rather an ocular evidence of 

 what probably occurs always during a silent discharge of elec- 

 tricity from a cloud without being perceived. The elevation, 

 my distance from the cloud, and the lowness of the sun, ena- 

 bled me to look upon it in circumstances very favourable for 

 observing changes in it by changes in its action upon light. I 

 should suppose, that these illuminated beams were portions 

 rendered more highly symmetrical by their electric state, and 

 capable of reflecting light which was quenched in other regions. 

 The superior symmetry of the whole to an uniform dense nim- 

 bus, seems to be indicated by the fact, that the rainbow or co- 

 loured ring formed in it when it traversed the region in which 

 such a display was possible, had its colours far more com- 

 pletely insulated and defined. The phenomenon was singu- 

 larly like the aurora ; and this much perhaps may be inferred 

 respecting both, that as those whowereinthe region of this cloud 

 could certainly not see its changing beams, so those who were 

 in the region of an aurora could not see its streamers posses- 

 sing the aspect which they exhibit to those who see a vertical 

 projection of them. 



Dundee, Mai/ 26, 1829. 



