MOUNTAIN SLIDES. 285 
it both from disintegrating frost and from sudden drenching 
and dissolution by heavy showers, are gone, it is easy to see 
that, in a climate with severe winters, the removal of the for- 
est, and, consequently, of the soil it had contributed to form, 
might cause the displacement and descent of great masses of 
rock. The woods, the vegetable mould, and the soil beneath, 
protect the rocks they cover from the direct action of heat and 
cold, and from the expansion and contraction which accompany 
them. Most rocks, while covered with earth, contain a con- 
siderable quantity of water.* A fragment of rock pervaded 
with moisture cracks and splits, if thrown into a furnace, and 
speak of a similar use of this numeral in more important cases—have often 
been understood as expressions of a known number, when in fact they mean 
simply many. The number “‘ fifteen thousand” has found its way to Rome, 
and De Quincey seriously informs us, on the authority of a lady who had 
been at much pains to ascertain the evact truth, that, including closets large 
enough for a bed, the Vatican contains fifteen thousand rooms. Any one who 
has observed the vast dimensions of most of the apartments of that structure 
will admit that we make a very small allowance of space when we assign a 
square rod, sixteen and a half feet square, to each room upon the average. 
On an acre, there might be one hundred and sixty such rooms, including parti- 
tion walls; and, to contain fifteen thousand of them, a building must cover 
more than nine acres, and be ten stories high, or possess other equivalent 
dimensions, which, as every traveller knows, many times exceeds the truth. 
The value of a high standard of accuracy in scientific observation can 
hardly be overrated; but habits of rigorous exactness will never be formed 
by an investigator who allows himself to trust implicitly to the numerical 
precision of the results of a few experiments, The wonderful accuracy of 
geodetic measurements in modern times is, in general, attained by taking the 
mean of a great number of observations at every station, and this final pre- 
cision is but the mutual balance and compensation of numerous errors. 
The pretended exactness of statistical tables is too often little better than 
an imposture ; and those founded not on direct estimation by competent ob- 
servers, but on the report of persons who have no particular interest in know- 
ing the truth, but often have a motive for distorting it, are commonly to be 
regarded as but vague guesses at the actual fact. 
* Rock is permeable by water to a greater extent than is generally sup- 
posed. Freshly quarried marble, and even granite, as well as most other 
stones, are sensibly heavier, as well as softer and more easily wrought, than 
after they are dried and hardened by air-seasoning. Many sandstones are 
porous enough to serve as filters for liquids, and much of that of Upper Egypt 
