40   msronrnrz. _
which his opinions are founded. The only purpose and effect of -such
testimony is to enlighten the minds of a jury upon subjects upon which
they have acquired no theoretical or practical knowledge. A witness
who has acquired knowledge in a particular art by long experience, can
generally give a more intelligent, intelligible, and satisfactory opinion
about his vocation than can be given by scientiuc experts upon questions
of science,.upon which opinions are often formed by means of theories,
conjectures, and abstract reasonings.
Upon subjects of general knowledge, and upon matters pertaining to
the daily occurrences and the usual business avocations of ordinary life,
ajury is presumed to be familiar, and to need no aid from the opinions
of a witness in making necessary and natural inferences and deductions
from facts ascertained by them from the evidence offered. When such
subjects and matters are to be investigated upon a trial, the rules of evi-
dence as to:expert witnesses are not applicable. V But in a case like the
one now before us, where they are required to consider the construction,
. qualityycapacity, and proper operation of complicated machinery, they
must be informed in some way upon the subject, as but few jurors are
qualified to form correct conclusions as to an art with which they are
not familiar. ._:··The reasonable andlegal way of obtaining such necessary
information is the hearing of the opinions of witnesses who, by the usual
methods of acquiring such knowledge and skill, have made themselves
capable of forming and expressing, intelligent and rational views upon `
such subjects. ~ V   . - ~ —,
In this case the jury on the trial might have been able to decide that
the saw-mill would not exert the full capacity of itspower, and do good
work, from the facts? stated by the witnesses that the saw was warped,
and the frame of the machinery was out of plumb, and was operated by
persons who hadlittle knowledge and skill in such business; but they
would not have been able to determine from such facts the questions
arising out of the express warranty as to the exact horseepower of the
engine furnished by the plaintiffs, in alleged compliance with the terms
of their engagement, and the reasonable and proper management cove-
nanted by the defendants; and these were some of the material questions
in issue to be determined by the jury. .
I A steam saw-mill is certainly a kind of machinery that requires pe-
culiar knowledge and skill in its erection and operation, in order to ex-
ert and exercise the full capacity of its power. I think that I acted
right on the trial of this case in allowing an experienced machinist, who
had acquired information by actual observation and careful examination,
to give his opinion in evidence to enlighten the minds of the jury as to
the horse-power of the engine, and whether the saw-mill machinery was
in such working order as to exert the full force of its power; and also
upon the question as to what would be the effects and consequences that
would result from operating the machinery when not properly adjusted,
‘ and unskillfully managed. _
, I will now consider more particularly the exceptions of thedefendant,
as stated in the brief of their counsel. First exception: