2G0 
SEDIMENT OF THE PO. 
essentially change the line of junction between plain and 
mountain, and carry back a long reach of the Adriatic coast 
many miles to the west.* 
It is, indeed, not to be supposed that all the degradation 
* I do not use the numbers I have borrowed or assumed as factors the 
value of which is precisely ascertained; nor, for the purposes of the 
present argument, is quantitative exactness important. I employ numeri¬ 
cal statements simply as a means of aiding the imagination to form a 
general and certainly not extravagant idea of the extent of geographical 
revolutions which man has done much to accelerate, if not, strictly speak¬ 
ing, to produce. 
There is an old proverb, Dolus latet in generalibus , and Arthur Young 
is not the only public economist who has warned his readers against the 
deceitfulness of round numbers. I think, on the contrary, that vastly 
more error has been produced by the affectation of precision in cases where 
precision is impossible. In all the great operations of terrestrial nature, 
the elements are so numerous and so difficult of exact appreciation, that, 
until the means of scientific observation and measurement are much more 
perfected than they now are, we must content ourselves with general ap¬ 
proximations. I say terrestrial nature, because in cosmical movements we 
have fewer elements to deal with, and may therefore arrive at much more 
rigorous accuracy in determination of time and place than we can in fixing 
and predicting the quantities and the epochs of variable natural phenomena 
on the earth’s surface. 
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 precision is but the mutual balance and compensation of numerous 
errors. 
Travellers are often misled by local habits in the use of what may be 
called representative numbers, where a definite is put for an indefinite 
quantity. A Greek, who wished to express the notion of a great, but un¬ 
determined number, used “myriad, or ten thousand;” a Roman, “six 
hundred ; ” an Oriental, “ forty,” or, at present, very commonly, “ fifteen 
thousand.” Many a tourist has gravely repeated, as an ascertained fact, 
the vague statement of the Arabs and the monks of Mount Sinai, that the 
ascent from the convent of St. Catherine to the summit of Gebel Moosa 
counts “ fifteen thousand ” steps, though the difference of level is barely 
two thousand feet, and the “Forty” Thieves, the “forty” martyr monks 
