20 THOMSON. — ATMOSPHEKIC ELECTRICITY. 



dent existence ? What relation, if any, have they to clouds ; and 

 if related, to what kind of clouds are they related ? 



Pulsations. — Do pulsations travel in any invariable direction ? 

 What time do they take to get from one part of the heavens to 

 another ? Are they running sheets of continuous light, or fixed 

 patches which become luminous, or more luminous, in rapid suc- 

 cession ; and if patches, do these appear to be foreshortened 

 streamers ? Are the same patches luminous in successive pulsa- 

 tions ? 



Sounds (?) — As some have suspected the Aurora to be accom- 

 panied by sound, the observer's attention should be directed to 

 this question when an Aurora is seen during a calm. If sound be 

 suspected, the observer should endeavour by changing his posi- 

 tion, brushing off spicules of ice from the neighbourhood of his 

 ears, his whiskers, &c., to ascertain whether it can be referred to 

 the action of such wind as there is on some part of his dress or 

 person. If it should clearly appear that it is not referable to the 

 wind, then the circumstances of its occurrence, its character, its 

 relation (if any) to bursts of light, should be most carefully 

 noted. 



These questions are proposed merely to lead the observer to 

 direct his attention to various features of the phenomenon. 

 Answers are not demanded, except in such cases as definite 

 answers can be given ; and the observer should keep his attention 

 alive to observe and regard any other features which may appear 

 to be of interest. It is desirable that drawings should be made of 

 remarkable displays. 



Observations with Sir William Thomson's electrometer would 

 be very interesting in connexion with the Aurora, especially a 

 comparison of the readings before, during, and after a passage of 

 the Aurora across the zenith. 



IV.— ATMOSPHERIC ELECTRICITY. 



1. Instructions for the Observation of Atmospheric 

 Electricity. By Professor Sir William Thomson, 

 LL.D., F.RS. 



The instrument to be used is the portable electrometer described 

 in Sir Wm. Thomson's reprint of " Papers on Electrostatics and 

 Magnetism," §§ 368-378.* Full directions for keeping tiie instru- 

 ment in order, preparing it for use, and using it to make observa- 

 tions of atmospheric electricity, are to be found in sections 372- 

 376 ; these are summarized in the following short practical 

 rules : — 



I. The instrument having been received from the maker with 

 the inner surface of the glass, and all the metallic surfaces within, 



* A copy of this book has been sent by the author for the use of the officer 

 or officers to whom the observations of atmospheric electricity are committed. 



