ator diagram, the first thing to be
done is to allow the barrel to make two or three reciprocations with the
pencil resting against it, before opening the cock attached to the
cylinder. There will thus be traced a horizontal line, which is called the
_atmospheric line_, and in condensing engines, a part of the diagram will
be above and a part of it below this line; whereas, in high pressure
engines the whole of the diagram will be above this line. Upon this line
the vertical ordinates may be set off at equal distances, or upon any base
line parallel to it; but the usual course is to erect the ordinates on the
atmospheric line.

239. _Q._--Will you give an example of an indicator diagram?

[Illustration: Fig. 37]

_A._--Fig. 37 is an indicator diagram taken from a low pressure engine, and
the waving line _a b c_, forming a sort of irregular parallelogram, is that
which is described by the pencil. The atmospheric line is represented by
the line o o. The scale at the side shows the pressure of the steam, which
in this engine rose to about 9 lbs. per square inch, and the vacuum fell to
11 lbs. The steam begins to be cut off when, about one-fourth of the stroke
has been performed, and the pressure consequently falls.

240. _Q._--Is this species of indicator which you have just described
applicable to locomotive engines?

_A._--It is no doubt applicable under suitable conditions; but another
species of indicator has been applied by Mr. Gooch to locomotive engines,
which presents several features of superiority for such a purpose.

This indicator has its cylinder placed horizontally; and its piston
compresses two elliptical springs; a slide valve is substituted for a cock,
to open or close the communication with the engine. The top of the piston
rod of this indicator is connected to the short arm of a smaller lever, to
the longer arm of which the pencil is attached, and the pencil has thus a
considerably larger amount of motion than the piston; but it moves in the
arc of a circle instead of in a straight line. The pencil marks on a web of
paper, which is unwound from one drum and wound on to another, so that a
succession of diagrams are taken without the necessity of any intermediate
manipulation.

241. _Q._--These diagrams being taken with a pencil moving in an arc, will
be of a distorted form?

_A._--They will not be of the usual form, but they may be easily translated
into the usual form. It is undoubtedly preferabl