. 
. 
180 Remarks on the Geological Sketch of Cumberland, &c. 
«© The data from which Dr. W. infers the dimensions of so 
small a wire may serve as a means of estimating the accuracy 
of M. Prony’s method when applied fo the measure of such ob- 
jects. 
<¢ A wire of pure platina is drawn till ten grains of it mea- 
sure 24 inches, so that its diameter is thus known to be +1,dth 
of an inch. 
<¢ A portion of this wire is then coated with silver cast round 
it in a cylindrical mould, (about 3,ths of an inch in diameter). 
«¢ The cylinder is then drawn till each inch is elongated to 
400 inches, in which state the diameter of the platina is known 
to be reduced in the proportion of the square root of 400 or 
twenty-fold: so that its diameter is then ~34,,dth of an inch. 
“< If any portion of the silver wire be then further drawn till 
one inch measures nine inches, the platina wire within it is then 
reduced to 4d part of its Jast diameter, and is consequently 
eoeodth of an inch in thickness. 
“¢ If the silver wire be then dissolved by nitric acid, the dia- 
meter of the platina which remains undissolved (although kept 
perfectly clean) could not with confidence be pronounced inac- 
curate by a mensuration in which its dimensions were at first 
presumed to be yi5,dths of an inch. 
Feb. 20, 1816. «© W.H. Woxtaston.”’ 
XXVIII. Remarks on the Geological Sketch of a Part of Cum- 
berland and Westmoreland. 
To Mr. Tilloch. 
Sm, — Ix your Magazine for lust month, (p.41,) in an article 
entitled “‘A Geological Sketch of a Part of Cumberland and West- 
moreland,’’ your correspondent notices as a fact, that carbonate 
of strontites has been found in the basalt of the Giant’s Cause- 
way. This is a circumstance that has frequently been noticed 
to me, and specimens have been shown me, as carbonate of 
strontites, which [ have uniformly found to be carbonate of lime. 
It is nothing uncommon to mistake the one substance for the 
other, the carbonate of strontites from Braunsdorf in Saxony 
having long been considered as the hard carbonate of lime among 
the German mineralogists.—I should therefore be glad to know 
from your correspondent, through the medium of your Magazine, 
whether the specimen he refers to has been submitted to ana- 
lysis. 
Your correspondent likewise notices that the clay-slate and 
horublend slate of Skiddaw rest immediately on granite; which 
would 
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