Meteorological Essays and Observations. 347 
long enough zn vacuo, to disengage the air with which it is mixed, 
The consequence is, that it is liable to become liberated, and, of 
course, interferes with the results. ‘The original directions of the in- 
ventor have also been departed from, as to the proportions of the dif- 
ferent parts, and as to the construction of the indices. Those upon 
Rutherford’s plan are universally sealed with air in their upper parts, 
which acts as a spring against the expansion of the column: the iron 
index of one is liable thereby to become oxidated, and adheres to the 
glass when the mercury passes it, and it becomes entangled ; while 
the spirit of the other being unavoidably mixed with air, when the 
pressure is decreased by cold it is disengaged. ‘The air may be again 
dissolved by increasing the pressure before a fire, and passing the 
bubble backwards and forwards, and, in a state of solution it does not 
appear to interfere with the equability of the expansion. This, how= 
ever, is not certain; and, at all events, it is liable to re-appear, 
and is very troublesome. These imperfections are by no means 
necessary consequences of the construction of the instruments, 
although the makers are very willing that they should be so con- 
sidered; but it requires great care and attention to guard against 
them, The general mounting of the meteorological thermometers is 
exceptionable in every way; buried as they are in a thick mass of 
wood, and covered with a clumsy guard of brass, they can but very 
slowly follow the impression of atmospheric temperature. The 
establishment of a perfect standard thermometer, which shall be 
accessible to all who may wish to consult it, will also, doubtless, be 
another object of the Committee of the Royal Society.” P, 368. 
Attention to the perfection of instruments, however, as the author 
has very correctly observed, will be all in vain, without a proper de- 
gree of care and system in making and recording the observations. 
The proper hours of the day for observation are indicated by the 
barometer; the maximum height of the mercurial column is at about 
nine A. M., the mean at twelve, and the minimum at three P, M. 
Where an individual has time to make three observations in the day, 
these hours should be preferred ; if he can only ebserve twice, the 
first and last hours should be the periods; and if only once, noon 
should be the time. Even those who merely consult the barometer 
as a weather-glass, would, Mr. Daniell asserts, find it an advantage to 
attend to those hours; for he has remarked that much the safest 
prognostications from this instrument may be derived from observing 
when the mercury is inclined to move contrary to its periodical 
course. If the column rise between nine A. M. and three P. M., it 
indicates fine weather ; if it fall from three to nine, rain may be ex- 
pected. 
The thermometer should be inspected at the same periods, in ad- 
dition to which the author recommends that the maximum and mini- 
mum, by register thermometers, should be carefully noted ; the instru- 
ments should, of course, be sheltered from every kind of radiation. 
