176 Mb. J. Bbtce on the Recent Progress of the 



the latter. It has been lately detected in many cases by Mr. Manuel 

 J. Johnson, at the Oxford Observatory, by means of the admirable 

 contrivances there adopted for automatic registration by photography. 

 But these have been so recently established that we must await the 

 result of more extended observations before attempting to generalize. 

 Sir John Herschel has expressed his decided opinion (EncT/clopcedia 

 Britannica, new edition, vol. xiv.), in opposition to most meteoro- 

 logists, that lightning is a consequence and not a cause of sudden 

 precipitation of large quantities of rain and hail. " The utmost 

 amount of electrical agency which we can conceive influential in deter- 

 mining precipitation is the sudden relief of tension, that is density, on 

 the discharge of a flash, which, aided by the vibration of the thunder- 

 clap, may favour the coalescence of globules into drops, which otherwise 

 would have been kept asunder by their mutual repulsion. As a chemical 

 and magnetic agent, electi'icity is important ; but it can only produce 

 such atmospheric movements as are merely molecular." The negative 

 electricity produced during rainfalls, Faraday has shown to be caused 

 by the friction of water-drops against the substance rubbed. The same 

 is found in the spray of waterfalls, even at several hundred feet dis- 

 tance. Eespecting clouds, Mr. Drew, in his late excellent little work 

 on Practical Meteorology (Van Voorst, 1855), thus writes — " We are told 

 by some that they are vesicular vapour ; this is simply a hypothesis ; 

 we can only affirm with certainty that they consist of particles of 

 aqueous vapour in a peculiar state of aggregation, and that they float 

 in the lower regions of the atmosphere. That electricity affects their 

 state is pretty certain ; but facts are wanting on which to found a theory 

 as to its mode of operation." 



(19.) In this state of uncertainty, the observations and experiments of 

 Dr. A. Waller, of Kensington [Philosophical Transactions, 1847. Part I.), 

 are a welcome gift to meteorologists. He appears to have succeeded in 

 showing that vapour consists of globules or spherules, without any inter- 

 nal cavity ; that the minutest component molecules are not vesicular, 

 but water to the centre. He examined the globules by the microscope in 

 various ways. The ova-bearing filaments of the spider's web, and the 

 cocoon of the silkworm, were exposed to the steam of boiling water, 

 which was condensed upon them. In fogs also the web, covered with 

 globules, was removed, and fixed between small glass plates. Another 

 method employed was to cover a slip of glass with a thin coating of 

 Canada balsam, and to breathe on it. The moisture was found to 

 remain for many hours in minute globules on the balsam, and also to 

 sink into it below the surface. The forms were irregular — not perfect 

 spheres. In both cases the globules coalesced, and formed larger ones, 



