---
-----------------+--------------++-------------------+--------------
SPEED OF WIND | PRESSURE || SPEED OF WIND | PRESSURE
-----------------+--------------++-------------------+--------------
5 Miles per hour | 2 oz. || 35 miles per hour | 6 lb. 2 oz.
10 " " | 8 " || 40 " " | 8 "
15 " " | 1 lb. 2 " || 45 " " | 10 " 2 "
20 " " | 2 " || 50 " " | 12 " 2 "
25 " " | 3 " 2 " || 55 " " | 15 " 2 "
30 " " | 4 " 8 " || 60 " " | 18 "
-----------------+--------------++-------------------+--------------

VARYING DEGREES OF PRESSURE.--It is curious to notice how the increase
in speed changes the pressure against the blade. Thus, a wind blowing 20
miles an hour shows 2 pounds pressure; whereas a wind twice that
velocity, or 40 miles an hour, shows a pressure of 8 pounds, which is
four times greater than at 20 miles.

It differs, therefore, from the law with respect to water pressure,
which is constant in relation to the height or the head--that is, for
every 28 inches height of water a pound pressure is added.

POWER FROM WAVES AND TIDES.--Many attempts have been made to harness the
waves and the tide and some of them have been successful. This effort
has been directed to the work of converting the oscillations of the
waves into a rotary motion, and also to take advantage of the to-and-fro
movement of the tidal flow. There is a great field in this direction for
the ingenious boy.

A PROFITABLE FIELD.--In no direction of human enterprise is there such
a wide and profitable field for work, as in the generation of power. It
is constantly growing in prominence, and calls for the exercise of the
skill of the engineer and the ingenuity of the mechanic. Efficiency and
economy are the two great watchwords, and this is what the world is
striving for. Success will come to him who can contribute to it in the
smallest degree.

Capital is not looking for men who can cheapen the production of an
article 50 per cent., but 1 per cent. The commercial world does not
expect an article to be 100 per cent, better. Five per cent. would be an
inducement for business.

CHAPTER XII

ON MEASURES

HORSE-POWER.--When work is performed it is designated as horse-power,
usually indicated by the letters H. P.; but the unit of work is called a
_foot pound_.

If one pound should