icitude and
anxious investigation. Hence, notwithstanding his attempt to detail
his personal narrative from the learned works which contain the result
of his scientific researches, he has by no means succeeded in
effecting their separation. The ordinary reader, who has been
fascinated by his glowing description of tropical scenery, or his
graphic picture of savage manners, is, a few pages on, chilled by
disquisitions on the height of the barometer, the disk of the sun, or
the electricity of the atmosphere; while the scientific student, who
turns to his works for information on his favourite objects of study,
deems them strangely interspersed with rhapsodies on glowing sunsets,
silent forests, and sounding cataracts. It is scarcely possible to
find a reader to whom all these objects are equally interesting; and
therefore it is scarcely to be expected that his travels, unrivalled
as their genius and learning are, will ever be the object of general
popularity.

In truth, here, as in all the other branches of human thought, it will
be found that the rules of composition are the same, and that a
certain _unity of design_ is essential to general success or durable
fame. If an author has many different and opposite subjects of
interest in his head, which is not unfrequently the case with persons
of the higher order of intellect, and he can discant on all with equal
facility, or investigate all with equal eagerness, he will do well to
recollect that the minds of his readers are not likely to be equally
discursive, and that he is apt to destroy the influence, or mar the
effect of each, if he blends them together; separation of works is the
one thing needful there. A mathematical proposition, a passage of
poetry, a page of history, are all admirable things in their way, and
each may be part of a work destined to durable celebrity; but what
should we say to a composition which should present us, page about,
with a theorem of Euclid, a scene from Shakspeare, and a section from
Gibbon? Unity of effect, identity of train of thought, similarity of
ideas, are as necessary in a book of travels as in an epic poem, a
tragedy, or a painting. There is no such thing as one set of rules for
the fine arts, and another for works of thought or reflection. The
_Iliad_ is constructed on the same principles as the _Principia_ of
Newton, or the history of Thucydides.

What makes ordinary books of travels so uninteresting, and, in
general, so shor