190 PROCEEDINGS OF THE AMERICAN ACADEMY 



parisons, and the differences which are given in the last column mani- 

 fest, by the signs of plus and minus, the irregularities of the instrument, 

 and the error to be expected from these irregularities in single obser- 

 vations. I have arranged the same observations in Table III. according 

 to the sign and the value of the differences. From the sign of the 

 differences it appears that, when the barometers fall, the aneroid falls 

 most, and when the barometers rise, the aneroid rises most. In other 

 words, the aneroid index, moving either way from the place where it 

 agrees with the reading of the mercurial baromete-r, moves too fast. 

 The experiments with the air-pump indicate the same tendency more 

 unequivocally. For, in those experiments, where the two barometers 

 were moving in a direction which corresponds to a depression of the 

 common barometer, the aneroid always moved the most, so that when 

 the motion of the mercury in the pump-gage is subtracted from the 

 motion of the aneroid index, the sign is always plus ; at least, until 

 we approach the lower limit of range. Although this is the general 

 character of the differences, a nice examination of the observations 

 shows that here, as well as in the experiments with the air-pump, there 

 are errors and fluctuations which cannot be traced to 'any law of the 

 instrument, and against which no provision can be made. Table IV. 

 contains a series of observations made with the view of ascertaining 

 the stability of the levers in the aneroid barometer, and the firmness of 

 other parts of the instrument. The instrument was first read off; 

 and then, after being exposed to diminished pressure, it was noticed 

 with what fidelity and despatch the index returned to its original 

 position when the original pressure was restored. 



" It must not be forgotten that it is single observations, indicating 

 momentary changes of atmospheric pressure, on which the navigator 

 most relies. In some of the hurricanes to which he is exposed, the ba- 

 rometer occasionally sinks so low as to come within the range of those 

 experiments made with the air-pump. And yet here, if anywhere, 

 the aneroid barometer finds its appropriate sphere. In meteorology, 

 the barometer is the most important instrument of research. The ba- 

 rometer alone, of all the instruments employed in this research, is 

 independent of merely local changes, and gages the atmosphere to its 

 upper limit. But the range of atmospheric pressure is so limited, 

 that laborious series of observations, with the nicest barometers that 

 can be constructed, are necessary in order to develop the harmonies 



