in the substitution of the metric for the inch scale or vice versa.[1] When records have covered considerable periods of time a change of either to the other results not only in confusion but in positive loss.
Barometer Observations and Records.—Weather Bureau barometer records are made at 8:00 a.m. and 8:00 p.m.. Observers usually note any changes that may have occurred during the day. Making an observation for record that shall meet the demands for reasonable accuracy requires a certain amount of experience and familiarity with the barometer.
Inasmuch as temperature, pressure, humidity and wind observations are to be taken at clock time, and all these require about ten minutes in the aggregate, it is sometimes necessary to decide quickly as to preference of order. During heavy storms a variation in barometric pressure may change visibly; and in winter weather, temperature may rise more than 1 degree between 8:00 a.m. and 8:10 a.m. Judgment and experience must determine. As a rule, however, two minutes will be a generous allowance of time for temperature observation. To make such observations habitually out of the established time should be a good reason for looking with suspicion upon the records thus made; if for any reason an observation is made out of time, the fact and the time should be noted. Slipshod practise in the time of making observations may not impair the results, but they certainly impair the character of the observer.
Because body warmth may affect the attached thermometer, the temperature should first be noted. It is best to record the temperature to the nearest half-degree. In field work, especially, if the temperature is within a few degrees of the freezing point, reading the temperature to the nearest degree will be sufficient for ordinary determination. Below 29° F the temperature corrections are additive; above 28° F they are subtractive.
The milled screw at the bottom of the case raises or lowers the mercury to the scale. When the surface of the mercury
- ↑ If a change from the English mercury-inch system should become desirable, the millibar scale would be considered preferable to any other so far proposed. At Greenwich, where the acceleration is 981.17 centimeters, the standard of pressure is 1,013,800 dynes; at Paris it is 1,013,600 dynes; in the United States (U. S. Coast Survey determination for Lat. 45°) the standard is 1,013,200 dynes, acceleration 980.62 centimeters or 32.16 feet.
