t apparatus in each installation. The following table shows
that, as regards the surface occupied, the advantage is again with the
gas generator and gas motor:

Steam Engine. Gas Motor.
Surface covered. 85 sq. m. 72 sq. m.
Surface exposed. 33 " 43 "
--- ---
Total surface. 118 " 115 "

The estimates of installation formed by Mr. Witz set forth the expense
relative to the capital engaged exactly at the same figure of 32,000
francs for a motive power of 75 effective horses. The expenses of
keeping in repair, interest, etc., summed up, show that the cost per day
of 10 hours is 47.9 francs for the steam engine and 39.6 for the gas
motor, say a saving of 8.3 francs per day, or about 2,500 francs for a
year of 300 days' work.

The gas motor, therefore, effects a great saving, while at the same time
occupying less space, consuming less water and operating just as well.

With Mr. Witz we cheerfully admit all the advantages that he so clearly
establishes with his perfect competency in such matters, but there still
remain two points upon which we wish to be enlightened. Are not the
starting up, the operation and the keeping in repair of a gas generator
actually more complicated and more delicate than the same elements of a
steam engine? Does not the poor gas manufactured in a gas generator
present, from a hygienic point of view, danger sufficiently great to
proscribe the use of such apparatus in many circumstances?

Such are the points upon which we should like to be enlightened before
unreservedly sharing Mr. Witz's enthusiasm, which, however, is
justified, economically speaking, by the magnificent results of the
experiments made by the learned engineer.--_La Nature_.

[Illustration: FIG. 3.--GAS MOTOR OF 100 INDICATED HORSE POWER.]

* * * * *

IMPROVED PNEUMATIC HAMMER.

We publish illustrations of a Thwaites suspension pneumatic power 1/2 cwt.
hammer of a new design, for planishing pipes and plates, for which we
are indebted to _Engineering_. As indicated in the perspective view
(Fig. 1) the mechanism is supported at the center of a cross girder
resting on two cast iron square pillars, box section, each bolted down
to the foundations by four 11/4 in. diameter bolts. The measurements of
these columns and girders are given in Figs. 2 and 3, the former an
elevatio