of the small screw in the back of the case—usually the only screw-head in sight. The index hand moves in the same direction that the screw-driver turns. The adjusting should not be used to set the index more than three-tenths of an inch. If the error is materially greater than this, it is better to have the adjustment made by an expert. When this cannot be done without delay, the error may be temporarily reduced, for convenience in reading, to even tenths of an inch. If it is desirable to have the index read to sea level reduction, it may be lifted off the stud and replaced as nearly as possible in the correct position. Any slight difference may then be taken up by the adjusting screw.
In moving the adjustment screw, one must take into consideration the position in which the instrument habitually rests. If a barometer which has been adjusted to a hanging position is laid upon its back, the reading changes several hundredths of an inch, and vice versa. It therefore must be held in its habitual position when the adjustment is made.
If the error in reading is two- or three-tenths, the observer must watch the readings for several days to ascertain if the adjustment has changed. If the index has been set forward it is apt to “creep” forward still further; if backward, the creeping will be backward. The reason therefor is not known with certainty. An aneroid taken to a higher elevation is apt to respond quickly to the reduced pressure; taken to a level materially lower, it may not respond so quickly.[1]
If an aneroid is in a proper condition, tapping the case with the finger will cause an instantaneous vibratory movement of the index which will settle each time to the same position. If it fails to recover its normal position, bring the instrument rather sharply down upon a chair cushion or cane seat; if the index fails to recover its position, or does not vibrate, a binding at the lever joints exists, and the instrument should go to the repair shop. The best test as to whether or not it is in good- ↑ An aneroid taken by the author from sea level to stations in Colorado varying from 8000 to 13,000 feet, responded promptly to the decrease in pressure on the outward trip. After it had been brought back to sea level, it registered an altitude of about 2000 feet. At the end of three weeks it still varied by nearly 0.3 inch from normal pressure. It was therefore sent to the manufacturer to be put in order. This illustration will apply in many instances; it does not apply to aneroids of the better class made at the present time.
