42 ~ rniuinnlu. meronrna. ’
care are involved in a trial in court, a jury may be properly aided in de-
termining such question by the opinion of an intelligent and practical
mechanic, as he is better instructed-and qualified to form a correct and
just opinion upon such questions than an inexperienced jury. Trans-
portation Dine v. Hope, 95 U.   297.
The other cited cases (Buxton v. Somerset Pottm·.s’ Works, 121 Mass. 446;
Hopkins v. Indianapolis, etc., R. O0., 78 Ill. 32; Koons v. St. Louis R. Co.,
65 Mo. 592) do not appear to bein point, as they involved questions
which were not within the technical limits of any science, art, or trade,
i and required no special skill or knowledge, but werematters of general
observation or common experience, which a jury was competent to deter-
mine.- s I
Second exception:
"_Tl1e defendants proposed to show that, when they went to plaintifEs’
agent ,(Van`Gi1der) to purchase, they informed him that they were not quali-
fied to judge of the capacity and quality of machinery, having had then no
experience m its management. They explained to him particularly the kind
of work they desired, to do, the 'character of the timber, and other circum-
stances material, and asked him to recommend the `machinery necessary;
that thereupon he did recommend the machinery they afterwards purchased, p
. and assured them that it was of the character. and had the capacity, to do
their work; that defendants having confidence in p1aintiifs’ agent, and rely-
ing upon his representations, they made the purchase; and that those repre-
sentations were false, were so known to he by the said agent, and were
made with theintent to deceive and induce the defendants to buy. This tes-
timony was¤rejected."   ’l‘-_ r f
The contractof sale was consummated by a contract signed and sealed
by the defendants, and contained various express stipulations on the
part of defendants, and an express warranty ofthe capacity and quality
of machinery on the part of plaintiffs, which will be set forth in terms in
a subsequent part of this opinion. This exception presents no difficulties
`requiring elaborate discussion, as the principles of law involved are well
~ settled in text writers, and in judicial decisions of the highest authority. _
1_ In Best v. Bank, 101 U. S., 93, we find the lawthus announced:
_ "No principle of evidence is better settled at the common law than that,
when persons put their contracts in writing, it is, in the absence of fraud, ac-
cident, or mistake, conclusively presumed that the whole engagement, and the
extent and manner of their undertaking, was reduced to writing. When par-
· ties, without any fraud or mistake, have deliberately put their engagements
in writing, the law declares the writing to be not only the best, but the only,
evidence of their agreement, and we are not disposed to relax the rule. It
lhas been found to be a wholesome one, and, now that parties are allowed to
testify in their own behalf, the necessity of adhering strictly to it is all the
more important. "
I need not refer to other authorities firmly establishing the familiar
general rule that parol evidence of contemporary oral agreements, com-
munications, or stipulations cannot be permitted to vary, qualify, or
contradict, or add to, or subtract from, the absolute terms of a written
contract. This general rule is subject to some modifications, but no real
exceptions, in courts of common law. I will refer ·to some of these