THE USE OF WEATHER INSTRUMENTS 189 



the other cold, are placed side by side where the air pressure 

 is exactly the same on each, the warm barometer will read the 

 higher, simply because its mercury is warmer. And, with the 

 same air pressure, any barometer will read higher on warm 

 days than on cold days. Therefore, in order to compare the 

 pressure on different days the barometer must always be kept 

 at the same temperature, or its reading must be corrected for 

 different temperatures. Also, in order to compare the pres- 

 sures at different places all the barometers used must be kept 

 at the same temperature, or else the barometer readings 

 must be corrected for temperature. It is much easier to cor- 

 rect the readings. This is done by subtracting the proper 

 amount for any temperature above 28J^F. Table IX 

 gives the temperature corrections used by the U. S. Weather 

 Bureau. 



215. Correcting the Barometer for Altitude. Since the 

 barometer shows the weight, or pressure, of the air, it is evident 

 that the higher we go up a mountain, or ascend in a balloon, the 

 less air there is above to press upon the barometer and the 

 lower the barometer will read. This fact was discovered 

 soon after Galileo found that air has weight. Pascal, a 

 Frenchman, heard of Torricelli's experiment in 1644. After 

 several years of experimenting, Pascal concluded that the 

 mercury in the tube would stand lower on a mountain top than 

 at its base. He carried a tube to the top of a high tower and 

 noticed a slight drop in the mercury column. He then asked 

 his brother-in-law, who lived near the Puy de Dome, a moun- 

 tain in southern France, to carry a barometer to the summit of 



DIRECTIONS FOR USING TABLE IX. Notice that the table has a 

 column for each half inch of the barometer from 24 to 31 in. At the 

 left side is a temperature column. If the barometer reads 29.42 in. and its 

 attached thermometer reads 68, find the barometer column with heading 

 nearest to 29.42 (in this case the 29.5 column); then follow that column 

 down to the horizontal line running across from "68." Where that 

 line crosses the barometer column you find "0.10." This means that 

 0.10 in. must be subtracted from the barometer reading as a correction for 

 temperature. The corrected reading is therefore 29.32 in, 



