ENGINE POWER COMPUTER.—A typical example of special slide rules is shown
in Fig. 29, which represents, on a scale of about half full size, the
author’s Power Computer for Steam, Gas, and Oil Engines. This, as will
be seen, consists of a stock, on the lower portion of which is a scale
of cylinder diameters, while the upper portion carries a scale of
horse-powers. In the groove between these scales are two slides, also
carrying scales, and capable of sliding in edge contact with the stock
and with each other.
This instrument gives directly the brake horse-power of any steam, gas,
or oil engine; the indicated horse-power, the dimensions of an engine to
develop a given power, and the mechanical efficiency of an engine. The
calculation of piston speed, velocity ratios of pulleys and gear wheels,
the circumferential speed of pulleys, and the velocity of belts and
ropes driven thereby, are among the other principal purposes for which
the computer may be employed.
[Illustration: FIG. 29.]
THE SMITH-DAVIS PIECEWORK BALANCE CALCULATOR has two scales, 11 feet
long, having a range from 1d. to £20, and marked so that they can be
used either for money or time calculations. The scales are placed on the
rims of two similar wheels and so arranged that the divided edges come
together. The wheels are mounted on a spindle carried at each end in the
bearings of a supporting stand. The wheels are pressed together by a
spring, and move as one.
To set the scales one to the other, a treadle gear is arranged to take
the pressure of the spring so that when the fixed wheel is held by the
left hand the free wheel can be rotated by the right hand in either
direction. When the amount of the balance has been set to the combined
weekly wage the treadle is released locking the two wheels together,
when the whole can be turned and the amounts respectively due to each
man read off opposite his weekly wage. The Smith-Davis Premium
Calculator is on the same principle but the scales are about 4 feet 6
inches long and the wheels spring-controlled. Both instruments are
supplied by Messrs. John Davis & Son, Ltd., Derby.
THE BAINES SLIDE RULE.—In this rule, invented by Mr. H. M. Baines,
Lahore, four slides carrying scales are arranged to move, each in edge
contact with the next. The slides are kept in contact and given the
desired relative movement one to the other, by being attached (at the
back), to a jointed parallelogram. On this principle which is of general
application, the inventor has made a rule for the solution of problems
covered by Flamant’s formula for the flow of water in cast-iron pipes:—V
= 76·28_d_^{⁵⁄₇}_s_^{⁴⁄₇}, in which _s_ is the sine of the inclination
or loss of head; _d_ the diameter of the pipe in inches and V the
velocity in feet per second. The formula Q = AV is also included in the
scope of the rule, Q being the discharge in cubic feet per second and A
the cross sectional area of the pipe in square inches.
FARMAR’S PROFIT-CALCULATING RULE.—The application of the slide rule to
commercial calculations has been often attempted, but the degree of
accuracy required necessitates the use of a long scale, and generally
this results in a cumbersome instrument. In Farmar’s Profit-calculating
Rule the money scale is arranged in ten sections, these being mounted in
parallel form on a roller which takes the place of the upper scale of an
ordinary rule. The roller, which is ¾in. in diameter, is carried in
brackets secured to each end of the stock, so that by rotating the
roller any section of the money scale can be brought into reading with
the scale on the upper edge of the slide and with which the roller is in
contact. This scale gives percentages, and enables calculations to be
made showing profit on turnover, profit on cost, and discount. The lower
scale on the slide, and that on the stock adjacent to it, are similar to
the A and B scales of an ordinary rule. The instrument is supplied by
Messrs. J. Casartelli & Son, Manchester.