gauge does not show the actual pressures, but only the difference between the actual pressures exerted on each leg of the gauge, and that the actual pressure of both the outside air and of gas at rest in a pipe decreases in proportion to their weight as the distance of the point of observation from the centre of the earth increases. The actual pressure of both air and gas at the top of the pipe will be less than at the bottom by an amount equal to the weight of a column of air or gas of unit area and of a height equal to the difference in elevation between the two ends of the pipe. As the gas is lighter than air, the weight of the column of gas will. be less than that of the column of air Therefore, the actual pressure of the gas will decrease less than the actual pressure of the air, and consequently the difference between the actual pressure of the gas and that of the air will increase with an increase in elevation. The lighter the gas, that is the lower its specific gravity, the smaller will be the decrease in its absolute pressure and the greater the increase in the gauge pressure for any given increase in elevation.

The amount of this increase can be obtained for any particular case by dividing the difference between the weight of a column of air with an area of one square inch and a height equal to the difference in elevation between the two ends of the pipe and that of a column of gas of the same area and height by the weight of a cubic inch of water. For a gas with a specific gravity of .700 and for a difference of elevation of 100' between the two ends of the pipe, the calculation would be as follows:

The height of the columns of air and gas will be 100' or 1,200", and as the area of the columns is one square inch the volume will be 1,200 cubic inches. A cubic foot of air weighs 0.076357 lbs.,

and a cubic inch weighs

.076357
1728

=.00004418. 1,200 cubic inches

of air then, weigh .00004418 X 1,200 = .053016 lbs. Since the specific gravity of the gas is .700, the column of gas will weigh 7/10 as much as the column of air, and the difference between the weight of the column of air and that of the column of gas will be equal to 3/10 the weight of the column of air, or to .053016 × .3 = .0159048. The weight of a cubic inch of water is .036 lbs. and

the increase in pressure will therefore be 0159048 .036

= .44". Thus

a gas with a specific gravity of .700 will show an increase in pressure of .044" for each 10' increase in elevation, or an increase of 1/10" for each 22.6' of rise.

In the same way we find that a gas with a specific gravity of .600 will show an increase in pressure of .059" for each 10' of rise, or an increase of 1/10" pressure for each 17′ of rise; gas with a specific gravity of. 500 will show an increase of .073" for each 10' of rise, or an increase of 1/10" for each 13.6′ of rise; gas with a specific gravity of .400 will show an increase of .088" for each 10' of rise, or 1/10" for each 11.3′ of rise, and gas with a specific gravity of .300 will show an increase of .103" for each 10' of rise, or an increase of 1/10" for each 9.7' of rise. (Trustees.)

6. The atomic weights of various elements are as follows:

Give the molecular weights of and the proportion by weight in which the various elements are contained in the following sub

Ans. Each molecule of Carbonic Oxide, CO, contains one atom of Carbon, C, and one of Oxygen, O, and its weight is equal to the sum of the weights of these two atoms. Therefore its molecular weight is equal to the sum of the atomic weights of Carbon, C, and Oxygen, O, or to 16+12=28. This shows that 16 lbs. of Oxygen, O, unite with 12 lbs. of Carbon, C, to form 28 lbs of Carbonic Oxide, CO, or putting it in another way that Carbonic 16 Oxide, CO, contains. or 57.14% by weight of Oxygen, O, and

28

Working in the same way the various substances named are found to possess the following molecular

7. Describe, with sketches, some form of Jet Photometer used for determining the candle power of gas, and
give your opinion as to the reliability of the results obtained by the use of such an instrument.

Ans. The Improved Lowe Jet Photometer is an example of the type of these photometers, which deter-
mines the candle power of gas from the pressure required to maintain at a fixed height a flame produced by
burning the gas through an orifice of a definite size. It consists essentially of a King's pressure gauge on top of
which is fixed a small steatite tip, with a fine circular orifice, through which the gas is burned as it issues from
the gas space of the gauge. The gauge is mounted in a wooden case with a glass door. On the back of the
case a horizontal line is marked at a height of 7" above the top of the tip and a piece of colored glass on the
door has its top edge at the same height. A dry governor is placed in front of the stop cock on the gas pipe
leading to the gauge, so that the pressure on the supply side of the stop cock is kept constant. The stop cock
has a long lever handle, the free end of which moves over a scale marked on a quadrant attached to the side
of the case, as shown on the cut. The gauge is made so as to give the least possible friction, and this in con-
nection with the close adjustment of the supply of gas, made possible by the long handle of the stop cock, ena-
bles the height of the flame to be regulated so that its tip is exactly in line with the marks on the door and the

Lowe Jet Photometer.

OPEN

back of the case as determined by sighting from one line to the other.

In the old form of these photometers the scale on the gauge is graduated to hundredths of an inch and the candle power, corresponding to the pressure which exists when the flame is adjusted to the proper height, is obtained from a table furnished with the instrument, either separately or in the form of an auxiliary scale placed below the pressure scale and properly tied into it. In the more modern forms there is no pressure scale as such, the semicircular scale over which the pointer of the gauge travels, being graduated to give the candle power by direct reading, as shown on the accompanying cut.

It is of course important that the gauge should show the correct pressure, and to do this the water line must be maintained at the proper level at all times. The adjustment of the water line is provided for by a small cylindrical tank mounted on the side of the case opposite to that on which the gas cock is placed. This tank, which communicates with the gauge by a connecting pipe, is provided with a piston, and the space below this piston being filled with water, the water line of the gauge can be readily adjusted to the proper level by moving the piston up or down according as the water is above or below the proper point.

Another style of jet photometer, of which the Jones Jet is an example, determines the candle power of the gas, from the varying height of a flame produced by burning the gas through a small orifice at a constant rate or at a constant pressure. The Jones Jet consists of a steatite tip with a fine circular orifice to which the gas is supplied by a volumetric governor and the candle power scale is marked on a colored glass chimney surrounding the flame.

If illuminating gas could be made at all times of a uniform composition, a jet photometer if once correctly adjusted would give fairly accurate absolute candle-power results. Since it is not possible to make such gas of a perfectly uniform composition such a photometer cannot be depended upon to give absolute results that are accurate, as even when the tip is of the proper size for the kind of gas to be tested and the readings are correct when the gas has a certain composition, any change in this composition will change the readings even though the illuminating value remains the same. It is obviously entirely out of the question to attempt, as is sometimes done, to use the same jet photometer without change of tip