196 Instructiotisjbr the Scientific Expedition 



moving in opposed or differing directions. In such cases, the sun, 

 moon, or a star should be taken as a point to fix the eye. In storms 

 the barometer should be very assiduously noted in relation to the 

 varying phases of the gale and the changes of vi'ind, and particularly 

 to those sudden shifts of wind which characterize revolving storms. 

 The Council are not aware that the state of the barometer during 

 " a white squall" has ever been very carefully noted from instant to 

 instant ; or that it, or the more sensitive sympiesometer, have been 

 referred to during the approach and recess of a waterspout. 



The phenomena of ordinary thunder-storms may be thought to 

 aiFord little matter for remark, and extraordinary ones will be noted 

 of course. Yet there is one point to which we should wish that 

 some attention might be paid, — it is the sudden gush of rain which 

 is almost sure to succeed a violent detonation immediately over-head. 

 Is this rain a cause or a consequence of the electric discharge ? Opinion 

 would seem to lean to the latter side, or rather, we are not aware 

 that the former has been maintained or even suggested. Yet it is very 

 defensible. In the sudden agglomeration of many minute and feebly 

 electrified globules into one rain drop, the quantity of electricity is 

 increased in a greater proportion than the surface over which (ac- 

 cording to the laws of electric distribution) it is spread. Its tension 

 therefore is increased, and may attain the point when it is capable 

 of separating from the drop to seek the surface of the cloud, or of the 

 newly-formed descending body of rain, which, under such circum- 

 stances, and with respect to electricity of such a tension, may be re- 

 garded as a conducting medium. Arrived at this surface, the tension 

 for the same reason becomes enormous, and a flash escapes. 



The following points should be observed, with a view to this 

 mode of regarding the formation of lightning. 1st. The actual 

 electric state of that rain which follows suddenly after a discharge 

 originating vertically over head. 



2nd. Does lightning ever happen without rain in the immediate 

 point where it originates, or at least without a rapid formation and 

 increase of cloud at that point ? 



3rd. Does it ever lighten from a cloud undergoing actual diminu- 

 tion from evaporation ? 



4th. Do the cumular clouds, already noticed as continually forming 

 and raining in the calm latitudes, usually or frequently send forth 

 flashes of lightning ; and if so, under what conditions, and with what 

 eff^ects ? 



Observations of Aurora will form a highly interesting subject, should 

 the Expedition be under the necessity of wintering, or of passing any 

 of the later part of the season in south latitudes admitting of their 

 exhibition. Their eff'ects on the magnetic needle will of course be 

 narrowly watched; but all their phenomena should be minutely 

 registered, such as the formation, colour, extent, situation, move- 

 ment, and disappearance of arches, patches, banks, and streamers. 

 In particular we would draw attention to an appearance which some- 

 times occurs, and which cannot but be regarded as highly instruct- 

 ive. It consists in pulsations propagated with more or less swift- 



