44 BULLETIN 102, UNITED STATES NATIONAL MUSEUM. 
When mantle burners are opened so as to admit more gas than is 
necessary, the familiar “hissing” or blowing sound is produced. 
This has, first, a tendency to break the mantle and chimney ; second, 
waste the gas ; and, third, lowers the candlepower of the lamp. The 
majority of natural gas consumers do not appreciate that gas burn- 
ers need care and attention and that periodic cleaning is absolutely 
essential if satisfactory results are to be obtained. 
The data given on page 40 show the marked differences in results 
that may be obtained in using natural gas in the fire pot of an ordi- 
nary coal furnace, as against a specially built natural gas furnace. 
BAROMETRIC CHANGES MAKE MORE DIFFERENCE ON TOTAL PRESSURE THAN GAGE 
PRESSURE VARIATION. 
On account of the changing atmospheric conditions, the barometric 
pressure varies from day to day and from hour to hour on the same 
day. Thus, the atmospheric pressure, at Louisville, Ky., on January 
21., 1918, was 30.47, and on January 11 was 29.19 inches, this differ- 
ence of 1.28 inches of mercury being the equivalent of 0.627 pound 
to the square inch, or 10 ounces to the square inch, or considerably 
more than the entire range of variation in gage pressure. 
ATMOSPHERIC TEMPERATURE CHANGES HEATING VALUE OF GAS MORE THAN CHANGES 
IN GAGE PRESSURE. 
The variation in mean monthly temperature of natural gas at 
Louisville, Ky., is shown on page 45. 
The variation in temperature of natural gas in the underground 
mains makes more difference in the heating value than the variation 
in gage pressure. The maximum fluctuation in temperature produces 
a difference in heating value of about 5 per cent, while the maxi- 
mum fluctuation in pressure produces a difference in heating value 
of less than 4 per cent. Furthermore, these variations work in oppo- 
site directions; that is, in winter time when the pressure is low, 
therefore tending to decrease the heating value, the temperature is 
low, tending to increase the heating value. This increase due to low 
temperature will always be more than the decrease due to low 
pressure. 
EFFECT OF PRESSURE OR TEMPERATURE CHANGES ON HEATING VALUE OF GAS. 
These will produce changes in volume, but will neither destroy nor 
create any heat units, and hence will neither increase nor decrease 
the total number of heat units contained in the gas. However, the 
volumetric changes will always alter the distribution of the total 
number of heat units, as follows : 
