or WATER MILLS, &e. 421 
the mutual ations and reactions of the parts on each other 
do not alter the motion of their common centre of gravity. 
[Newt. Princip. B. 1. Law 3. and Corol. 4.] 
Hence the increafe of power from the centrifugal force, 
multiplied into the augmentation of velocity thereby oc- 
eafioned, juft equals the force neceflary to move the water 
into its fpiral dire@tion ; which correfponds with what was 
before deduced from a different calculus. 
Note under the head Area of the Apertures, page 192, 
the refulting equation, which, by inadvertently ufing a 
AV AV 
for 2a, &c. is — fhould be ; and, of 
8.g24vh 18.47Vh 
courfe, the number 8.924, in rule 4th page 193, fhould be 
18.47; but this overfight does not affe&t any other part of 
the calculation. 
Philadelphia 4th 4mo. 1793. 
Wm WARING, 
N°. KAXV. 
An Improvement on metalic Conductors or Lightening-rods, 
ina Letter to Dr. DAvip RITTENHOUSE, Prefident of 
the Society, from RoBERT Patterson of Philadel- 
phia. 
&r This Paper was honoured with the Magellanic Pre~ 
mium, by an Award, of the Society in December 1792. 
Sk, 
Read Nov. J," R OM the inftances which now and then 
Be 79° occur of houfes being ftruck with lightening, 
that are turnifhed with metalic conductors, and the fre- 
VOL. UL Ss quent 
