230 
plete discussion of the facts as proof. He turns next 
to Espy’s conviction, that rain might be produced 
economically whenever it was wanted, and cites Pro- 
fessor Henry’s opinion in the matter: — 
“T have great respect for Mr. Espy’s scientific character, not- 
withstanding his aberration, in a practical point of view, as to 
the economical production of rain. The fact has been abun- 
dantly proved by observation, that a large fire sometimes pro- 
duces an overturn in the unstable equilibrium of the atmosphere, 
and gives rise to the beginning of a violent storm.” 
The opinion of Professor Everett, president of the 
Meteorological society, is also cited. He believed that 
great battles and great fires tend to produce rain, but 
that rain does not, of necessity, follow battles or fires. 
The climate of Australia being peculiar, Mr. Rus- 
sell has endeavored to collect the records bearing 
upon the question there; and, there having been no 
battles (except a mimic one, which produced no rain), 
he passes to an examination of the meteorological 
conditions of the times of the great fires which have 
occurred in Sydney since 1860, and assumes, that, if 
a fire produced rain, it would fall within forty-eight 
hours. His record embraces forty-two large fires 
(including two serious explosions), extending over a 
period of twenty-one years; and he concludes that 
there is not one instance in which rain has followed 
within the forty-eight hours as an evident conse- 
quence of the fire. 
In cases where it is asserted or believed that rain 
has been produced artificially, it would be interesting 
to examine whether the rain was due to the fires or to 
ordinary meteorological changes. While it is evident 
that some of the most competent authorities in Eng- 
land and America think that under certain circum- 
stances rain may be produced artificially, Mr. Russell 
thinks they all carefully avoided saying what the cir- 
cumstances were; and he proceeds to develop some 
idea of what they are, from a consideration of the 
natural conditions under which rain is deposited, 
and adducing certain instances as illustrations, from 
nature, of the conditions under which the leading 
scientific meteorologists of the day tell us that rain 
is formed. He says, — 
“Tf we can get a measure of these [observed] effects, it will 
serve as a guide in estimating what would be required to make 
rain. At Sydney the average relative humidity is 73, and at 
Windsor it is rather less; and we have just learned that such 
atmosphere lifted from Windsor to Currajong, 1,800 feet, depos- 
its 60 per cent more rain. If we could make it rise up over 
Sydney 1,800 feet, we might fairly expect to get 60 per cent more 
rain. Now, a wall built 1,800 feet high, and of considerable 
length, so that a wind would not divide and go round it, but go 
over, would have the desired effect; i.e., to lift the air and cause 
rain: but any thing that would do this would serve the purpose, 
and it may be done by fire; but of course the fire must have the 
effect of lifting the atmosphere up. It will not do for the prod- 
ucts of the fire to rise up slowly, mixing with the air, and mak- 
ing it drier as they rise. If itis to have the effect of a wall, — that 
is, making the whole of the air passing over rise up 1,800 feet, — 
. it must act as an explosion would do, suddenly, or by a constant 
uprush of such violence that it would rise up 1,800 feet. The 
force necessary to do this is easily computed, and we can in this 
way get a money value for the work to be done. At Sydney the 
average velocity of the wind is 11 miles per hour; and all the air 
passing over is to be lifted, and the weight of it on the surface 
is, say, 145 pounds on the square inch, and 13} pounds at 1,800 
feet high. At least, for our present purpose, these figures are 
. 
SCIENCE. 
[Vou. Ill. Newaams 
sufliciently exact, The average weight to be lifted, therefore, is 
14 pounds on the square inch. The fire must have the same 
length as the proposed wall, for the same reason, and a breadth 
equal to the forward motion ofthe air in a given time. We have 
therefore to lift a weight of 14 pounds on the square inch over 
a surface of 1,000 feet by 10 miles (52,800 feet), and raise it up 
1,800 feet every minute. To do this we will assume that coal is 
employed, and that, as it is burnt in the air, the whole of its heat 
will be effective. The mechanical equivalent of good coal is 
14,000,000 foot-pounds for each pound of coal used. We haye, 
therefore, — 
14 x 12 x 12 x 1,000 x 1,800 x 52,800 
14,000,000 x 112 x 20 
8,800,000 tons in a day, or nearly 9,000,000 tons of coal per day, 
to increase the rainfall 60 per cent, at a cost, at 10s. per ton, of 
£4,500,000. 
‘“‘Of course this is only a theoretical experiment, and ignores 
all the heat lost by radiation and imperfect combustion; but it 
serves to give some idea of what is necessary to disturb the 
course of nature, and, I think, shows how utterly futile any such 
attempt would be, even near the sea, where the air is moist.” 
= 6,110 tons per minute = 
It would seem unreasonable, Mr. Russell concludes, 
to hope for the economical production of rain under 
ordinary circumstances; and our only chance would : 
be to take advantage of a time when the atmosphere 
is in the condition called unstable equilibrium, or 
when a cold current overlies a warm one. If under 
these conditions we could set the warm current movy- 
ing upwards, and once flowing into the cold one, a 
considerable quantity of rain might fall; but this 
favorable condition seldom exists in nature. 
ROTATION OF JUPITER. 
Mr. W. F. DENNING has recently published an in- 
vestigation of the rotation of certain spots on Jupiter 
which confirms in a remarkable degree a theory al- 
ready propounded that this planet resembles the sun 
in not only rotating in different times in different 
latitudes, but in having the period of rotation of its 
equatorial region shorter than that of regions in mid- 
dle latitude. From the red spot which has formed so 
conspicuous an object on the planet for nearly five 
years, the following rotation periods are obtained at 
different times : — 
a 
Number of | Period of 
Interval, rotations. rotation. 
Dei MERE Sie 
1880, Sept. 27-1881, March17. . . . 413 9 55 35.6 
1881, July 8-1882, March30... . 640 9 55 38.2 
1882, July 29-1888, May 4... .| 674 9 55 39.1 
1883, Aug. 28-1883, Dec. Sse eae 251 9 35 38.8 
A gradual lengthening of the period is thus indi- 
cated. On the other hand, from a white spot near 
the equator the following times are obtained : — 
Number of | Periodof 
rotation., 
Toterval. rotations. 
1880, Oct. 20-1881, Sept. 30 
1881, Sept. 30-1882, Dec. 23. 
1882, Dec. 238-1883, Nov. 25. . 
