Prof. J. Lovering on the Aneroid Barometer. 253 
The experiments with the air-pump indicate the same tendency 
more unequivocally and to about the same proportional amount. 
in these experiments, where the barometer and the pump- 
atige were indicating the effect of diminished pressure, the ane- 
roid stood at the lowest point; so that when the elevation of the 
mercury in the pump-gauge is subtracted from the backward mo- 
tion of the index in the aneroid the sign is always plus: at least, 
until the lower limit of range is approached. 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 
traced to any law of the instrument, and against which no pro- 
vision can be made. 
able IV. contains a series of observations made with the view 
of ascertaining the stability in the levers of the aneroid barome- 
ter and the firmness of other parts of the instrument. The in- 
strument was read off before being exposed to diminished pres- 
sure: it was then noticed with what fidelity and dispatch the in- 
dex returned to its original position when the original pressure 
was restored, 1 
In estimating the merits of the aneroid barometer, it must not 
be forgotten that it is single observations, indicating momentary 
changes of the atmospheric pressure, on which the navigator most 
relies. In some of the hurricanes to which he is exposed, the 
barometer occasionally sinks so low as to come within the range 
of the experiments made with the air-pump. And yet here if 
any where the aneroid barometer finds its appropriate sphere. In 
meteorology, the barometer is the most important instrument of 
research. The barometer alone of all the instruments in the 
greater force to the application of the aneroid barometer to the 
measurement of heights above the level of the sea. An elevation 
of eighty-seven feet depresses the mercury by about -1 of an inch 
only; hence a small error in the barometer will entail a large 
error on the estimated elevation. Moreover, a very long series of 
observations will, in this case, be generally impracticable. I 
would suggest one farther consideration. ‘The mercurial barom- 
