s8i 
the mean density of the earth . 
bodies, the two ratios compared, must give the ratio of their 
densities, and which is therefore that of 17804 to 99 33, or 
1434 to 800 nearly, or almost as 9 to 5 ; and so much does 
the mean density of the earth exceed that of the hill. Con- 
sequently, if we know the density of the latter, we shall 
thence obtain that of the former. 
At the time when this computation was first printed, in the 
year 1778, the real density of the hill was unknown. It was 
only known that it consisted chiefly of very hard and dense 
rocks, much heavier than common stone, which is allowed to 
be 2-§- times the density of water. I then, by way of ex- 
ample in applying the density, multiplied by Qf, which pro- 
duced j- or 4-j for the density of the earth, on the smallest 
assumption ; till such time as we should come to know more 
nearly what the real density of those rocks is : and therefore 
I must feel reason to complain, that this number (4-) has 
often been stated, rather unfairly, as my final conclusion for 
the earth’s mean density ; instead of being only the very 
lowest limit that might be used, till we could better learn 
something on that point with more certainty. But a litho- 
logical survey of the mountain being afterwards accurately 
made, at my earnest request, by that excellent philosopher 
and geologist, Mr. Playfair, the result of which was pub- 
lished in the Philosophical Transactions for the year 1811 ; 
I then applied his mean statement of the rocks to my own 
calculations, which gave me the number 5 for the density of 
the earth ; as I published in the fourteenth volume of my 
edition of the Philosophical Transactions, and in the second 
volume of my Tracts. 
In Mr. Playfair’s account of the mountain, are given the 
