Professor Leslie on Electrical Theories . 
n 
it might take ten thousand years * *. Such are the vaunted per- 
formances of thunder-rods ! and such the advantages of their dif- 
ferent forms + ! Nor can we appeal to experience ; it never can 
be proved that thunder-rods have produced beneficial effects, 
but several instances may be cited where they have afforded no 
* This may bo determined by a very simple experiment. A brass ball, 1.7 
inch in diameter, and .1 inch thick, was heated to about 1400°, and held at the 
distance of half a foot from the conductor, for the space of 3 minutes, at the end 
of which it was cooled to 600°. Its effects seemed equal to that of a pointed wire 
held in the same situation. Hence the quantity of air which came in contact with 
the ball, was equal to what flowed upon the point. L,et m denote the mass of the 
ball, and x the excess of its temperature above that of the common air : then it is 
ascertained by experiment, that a diminution of temperature dx in the brass would 
• , • , m „ mdx ... 
communicate an equal rise to about — of the air, and therefore a portion — will 
10 L 10a; 
be heated up to the temperature of the brass, and be succeeded by other portions 
of air. 
But 
mdx 
10 x 
~ . Hyp. Log. *+C, and C — — . H. Log. 1400; 
whence the whole air brought, at different times, in contact with the ball is 
m 600 m 
“iQ* H. Log. Y4 oo’ or TO nearl y ) and consequently, in the space of 3 minutes, a 
sphere of air about 10 inches in diameter would flow upon the ball ; therefore in 
1000 years the pointed wire would exhaust the electricity from a sphere of air 
one furlong in diameter. I will not pretend that this calculation is entirely accu- 
rate ; but if it approach even within the thousandth part of the truth, it is more 
than sufficient for my purpose. 
*f- The utility of thunder-rods not being once questioned , it teas yet keenly disputed 
in England for several years , whether these should be terminated by points or by 
knobs. But, what is amusing , politics soon came to be mixed up with the controver- 
sy. The powder magazine at Pnrjleet , though guarded by pointed conductors , hap- 
pening , in 1778, to be struck by lightning , the Privy Council made an application 
to the Royal Society to investigate the cause of this accident. A Committee was 
accordingly named of its ablest members , who , still adhering to the hypothesis of 
Franklin , only recommended additional pointed conductors to be placed at nearer 
intervals. This Report , in the height of the revolutionary war , coidd not be other- 
wise than displeasing to the courtiers , who, from their violent antipathy to the Ame- 
rican philosopher , were as eager to depreciate his science as to deride his patriotism. 
They accordingly set on foot a subscription to enable Mr Wilson to perform electri- 
cal experiments on a large scale in the Pantheon , and the conclusions thence drawn 
seemed favourable to the theory of knobs. The Royal Society teas in consequence 
desired by high authority to revise their Report ; but the President , Sir John Pringle , 
replied with some warmth , that lie could not change the laws of nature ! This 
venerable person , however , being worried on all sides , soon resigned the chair in dis- 
gust , and retired into the country. 
