562 Murray on Atmospheric Electricity. 
off, and is composed of very attenuated silk-like filaments, above one inch 
in length. Unlike other crystals of ice, they possess a considerable degree 
of tenacity, and are more persisting than the hoar frost on other bodies. In 
position the filaments are divergent like radii from the pith of the twig, and 
have all the appearance of being produced, as Mr. Murray suspects, as jets 
from the external orifices of the wood, similar to the columnar masses of 
icicles which heave up the surface of damp peat-earthy ground in frosty 
weather. Whether this is an exemplification of the truth of M. Dutrochet’s 
theory, and of the counter currents of the sap in plants, is, however, not 
very obvious.* 
The incident of trees condensing the moisture of the air in foggy weather 
is noticed. Hence the author infers that thickly wooded countries must 
necessarily be colder and more humid than naked savannahs. ‘Trees are, 
therefore, it would seem, ready conductors of aerial electricity ; the climate 
being improved when woods are cleared away, and becoming more moist by 
planting. Of this circumstance there can be no doubt, as it appears from 
the histories of both this island and of North America. 
He alludes again to the curious phenomenon of certain spiders being en- 
dowed with the power of propelling threads, which, being acted on by posi- 
tive electricity, carries them aloft in the air, even against the direction of 
the wind. The author does this with a view, it would seem, of replying to 
Mr. Blackwall, who opposed the doctrine in ‘this Magazine ; but, from the 
many proofs brought forward by Mr. Murray and his friends, there seems 
to be no longer any doubt of the fact. Mr. Blackwall attributes their ascent 
to currents of warm air, which are ever rising in a greater or less degree 
from the earth. That such currents are always in action, notwithstanding 
Mr. Murray’s scepticism, is undoubtedly true ; but it is questionable whe- 
ther they are at all times so powerful as to cause the ascent of any particle 
of matter, even so light as an aeronautic spider. 
The author, in the seventh chapter, proceeds to describe the effects of 
atmospheric electricity, and the various phenomena of dew, rain, snow, &c. 
He espouses Dr. Wells’s theory of dew; not, it appears, from any experi- 
mental proof or convictions of his own, but from pure implicit accordance. 
That a still atmosphere is necessary to its formation in the open air, is most 
true, because agitation of the air accelerates evaporation; but that a clear 
nocturnal sky isalso necessary isa mistake. Dew is formed in every degree 
of temperature under one hundred degrees of Fahrenheit, and certainly is not 
a precipitation, otherwise it would fall on every body of equal temperature. 
Dead or sickly plants carry no dew, nor do those on dry places : dew will 
be copious within a hand-glass, or any other thick covering which obstructs 
the view of “a clear nocturnal sky ” and the agitation of the air, while not a 
drop is visible without. Healthy plants on every evaporating surface carry 
dew in calm weather. There also a lower degree of temperature exists ; 
but whether caused by evaporation, or radiation, or whether these terms 
are used synonymously, our author saith not. 
His remarks on meteorology in general bring him to the practical in- 
ferences deducible from his preceding observations, viz. that the severity 
of thunder and hail storms may be moderated by withdrawing a part of 
the atmospheric electricity by means of well constructed lightning-rods and 
paragréles. Aware of the diversity of opinion relative to the effects of these 
instruments, and incompetent to decide on which side the truth lies, we 
would only suggest (while we grant the plausibility of Mr. Murray’s con- 
clusions), whether if they be so efficient as to divest the storm-cloud 
of its dangerous effects, they should not only be used as temporary pro- 
tectors ? For, if effective at one time, so will they be at another. The 
* Since the above was written the highly extolled theory of M. Dutrochet 
has been proved incorrect, and abandoned by himself. (Foreign Review.) 
