Rules for connecting the temperature and elastic force of saturated steam.-Various formulae have been at different times propounded for deducing the elastic force of saturated steam from its temperature, and the temperature from the elastic force. The experiments of Mr. Southern, which were made at the instance of Boulton and Watt, led to the adoption of the following rules, which, though not quite so accurate as some others which have since been arrived at, are sufficiently so for practical purposes, and being intimately identified with engineering practice, it appears desirable to retain them.

THE TEMPERATURE OF SATURATED STEAM BEING GIVEN IN DEGEEES FAHRENHEIT, TO FIND THE CORRESPONDING ELASTIC FORCE IN INCHES OF MERCURY BY SOUTHERN'S RULE.

RULE.-To the given temperature add 51·3 degrees. From the logarithm of the sum subtract the logarithm of 135-767, which is 2.1327940. Multiply the remainder by 5.13, and to the natural number answering to the sum, add the constant fraction 1. The sum will be the elastic force in inches of

mercury.

Example.-If the temperature of saturated steam be 250.3° Fahrenheit, what will be the corresponding elastic force in Laches of mercury?

Here 250-3 × 513-301.6 Log. 2.4794313

135 767 Log. 2-1327940 subtract.

This natural number increased by 1 gives us 60.113 inches cimercury, as the measure of the elastic force sought.

THE ELASTIC FORCE OF SATURATED STEAM BEING GIVEN IN INCHES OF MERCURY, TO FIND THE CORRESPONDING TEMPERATURE IN DEGREES FAHRENHEIT BY SOUTHERN'S RULE.

RULE.-From the given elastic force subtract the constant fraction 1; divide the logarithm of the remainder by 5.13, and to the quotient add the logarithm 2.1327940. Find the natural number answering to the sum of the logarithms, and from the number thus found subtract the constant 51′3. The remainder will be the temperature sought in degrees Fahrenheit.

Example.-If the elastic force of saturated steam balances a vertical column of mercury 238.4 inches high, what is the temperature of that steam?

The temperature of the steam which will balance such a column of mercury, has been ascertained by observation to be 343.6 degrees.

TEMPERATURE AND PRESSURE OF STEAM.

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Experiments have been made by the French Academy, the Franklin Institute in America, and various other experimentalists, to determine the elastic force of steam at different temperatures; but of all these experiments, the most elaborate and the most widely accepted are those of M. Regnault. The results obtained by the French Academy are given in the following table, and those obtained by the Franklin Institute are very similar:

PRESSURE OF STEAM AT DIFFERENT TEMPERATURES.

Results of Experiments made by the French Academy.

An atmosphere is reckoned as being equal to 29.922 inches of mercury.

Formulæ for connecting the temperature and elastic force of steam have been given by Young, Tredgold, Prony, Biot, Roche, Magnus, Holtzmann, Rankine, Regnault, and many others—all more or less complicated. Regnault employs different formula

for different parts of the thermometric scale, as appears from the following recapitulation in which all the degrees are degrees centigrade :

REGNAULT'S FORMULA FOR THE TEMPERATURE AND ELASTIO FORCE OF STEAM.

which resembles the formula previously given by M. Biot. In this formula t is counted from 0° centigrade. a=4-7384380; Log. a1=0·006865036; Log. ß1=1.9967249; Log.b=2.1340339, and Log. c=0.6116485.

Between 100° and 230°, the formula he used is

Log. F—a—b at—cßt,

in which T=t+20, t being the centigrade temperature reckoned from 0°. Hence a 6.2640348; Log. a=1.994049292; Log. B=1.998343862; Log. b=0·1397743, and Log. c=0·6924351.

=

The principal properties of saturated steam as deduced from the experiments of M. Regnault, exhibiting the pressure, the relative volume, the temperature, the total heat, and the weight of a cubic foot of steam of different densities, are given by Mr. Clark in the following tables :