of Edinburgh, Session 1881-82 
801 
4. The Effect of Moisture on the Electric Discharge. By A. 
Macfarlane, M.A., D.Sc., and D. Rintoul, M.A. 
( Preliminary Notice.) 
In his recent lectures on Solar Physics,* * * § Professor Stokes, while 
expounding his theory of the connection of magnetic disturbances, 
aurorae, and earth currents, says : — “ We might not have tension 
enough to produce such a discharge (that is, a flash of lightning), 
the resistance to the passage of electricity from one portion of the air 
to another, which at any rate would be comparatively dry compared 
with what we have in warm latitudes, would prevent it by itself 
alone.” Professor Stokes subsequently remarked in a letter to 
Nature: f — “These words, without actually asserting, seem to 
imply that the resistance to such a discharge through moist air 
would he less than through dry. My attention has been called by 
a friend to the fact that it has been found by experiment that moist 
air insulates as well as dry. I have not met with experiments tending 
to show whether the resistance to a disruptive discharge is the same 
or not in the two. Be that as it may, it does not affect what 
follows ; for we know, as a fact, that thunderstorms are absent in 
high latitudes.” 
To the question here asked we have endeavoured to furnish an 
answer. The experimental method adopted was the same as that 
described in our paper on “ The Effect of Flame on the Electric 
Discharge.” £ The spark was taken between two brass discs, 
each 4 inches in diameter, supported parallel to one another inside 
a receiver, after the manner described in former papers. § The 
drying of the air was effected by first exhausting the moist air out 
of the receiver, and then allowing the air of the room to pass in 
slowly through one or more sulphuric acid tubes. In our first trials 
one drying tube was employed, but in the later trials two. Care 
was always taken that the air through which the discharge passed 
was at the atmospheric pressure. To obtain the humidity of the 
* Nature , vol. xxiv. p. 615. 
t Vol. xxv. p. 30. 
t Proc. R.S.E., 6tli March 1882, p. 567. 
§ Phil. Mag., Dec. 1880. 
