lve engine of half a horse power? A.
About 2 inches cylinder and 3 inch stroke, depending upon pressure
and velocity.

(7) R. W. H. writes: In a recent discussion on hot air and steam
portable engines it was decided to ask your opinion, which should be
final. Water is scarce, though enough to use steam is easily procured.
The country is hilly, so that lightness is desirable. The power wanted
is 6 horse, and movable, that is, on wheels. Which will be best, hot air
engine or steam engine? Which consumes most coal for a given power?
Which will be cheapest in above case? A. For small powers the hot air
engine is most economical, but we do not think it adapted to your
purpose. We would recommend the steam engine for a portable power.

(8) J. C. T. writes: 1. I have a water tank for supplying my boiler,
which is made of No. 22 galvanized iron; size 30 inches by 9 feet 4
inches. How many gallons will it hold? A. 342 gallons. 2. Will it be
better to have it painted inside? A. Yes. 3. How many years will the
tank wear under favorable circumstances, using well water? A.
Depends upon the care taken of it.

(9) W. H. C. asks: Is there any way of deadening the noise of machinery
overhead from the engine room below? The noise comes from machinery in
the weave room of an alpaca mill. A. This is generally accomplished by
setting the legs of the machines on thick pieces of India-rubber or
other non-conductor of sound.

(10) G. H. asks: How can I mount photos on glass and color them? A.
Take a strongly printed photograph on paper, and saturate it from the
back with a rag dipped in castor oil. Carefully rub off all excess from
the surface after obtaining thorough transparency. Take a piece of glass
an inch larger all round than the print, pour upon it dilute gelatin,
and then "squeegee" the print and glass together. Allow it to dry, and
then work in artists' oil colors from the back until you get the proper
effect from the front. Both landscapes and portraits can be effectively
colored by the above method without any great skill being required.

(11) C. W. S. asks: 1. Is there any practical and effective method known
for cutting screws by connecting the slide rest with the mandrel of the
lathe by gears or otherwise? A. This can be done in this way: attach a
spur wheel to the back of the face plate. Mount a similar wheel on a
short hollow shaft, and support the shaft by an arm bolted to the lathe
bed so that the two spur wheels will m