1 837. J 



Meteorological Observations. 



201 



nication of such observations addressed by channels as secure and as 

 little expensive as possible to the Secretary of this Institution, will be 

 considered as highly valuable. 



III. Of Meteorological Instruments, and first of the Barometer 

 and its attached Thermometer. 



The Barometer is the most important of all Meteorological instruments^ 

 Its office is to measure the actual pressure of the atmosphere on a given 

 horizontal surface at the time and place of observation. Its fluctuations 

 are observed to have considerable relation to changes in the weather,, 

 and especially of the wind. Hence its use as a weather-glass. 



A Barometer should be examined, before setting it up, for air-bubbles 

 in the tube, and for the existence of air above the mercury in the upper 

 part of the tube. This is done by gently inclining the instrument either 

 way from the horizontal position a little up and down ; when air-bub- 

 bles, if large, will be seen to run to and fro, and must be evacuated by 

 inverting the instrument and by gentle blows on it with the hand, 

 driving them up into the cistern. If this cannot be done, the instrument 

 is useless. If air exists to an objectionable amount above the quick- 

 silver, it will not tap sharp against the upper end of the tube when the 

 barometer is quickly inclined from a vertical position, so as to make 

 the mercury rise above its level, nearly to the top, and then gently 

 jerked lengthways and backwards. If the blow is puffy and dead, or is 

 not heard at a]l, the amount of air must be considerable, and may be 

 expelled by inversion. 



In fixing the barometer, choose a good light near a window, but not 

 exposed to sunshine, in a retired apartment, little liable to sudden 

 changes of temperature or to drafts of wind. Adjust the tube to a 

 vertical position by a plumb-line, and fix it so as never to shift from 

 that position. Before reading off, give a few taps on the instrument, 

 enough to make the upper end of the column of quicksilver shake visi- 

 bly, as the mercury is apt to adhere to the glass and give erroneous 

 readings. In reading, bring the index always opposite to one part- 

 The correct part to choose is the summit of the convexity of the 

 mercury, to which the index should be made a tangent, but if this be 

 difficult to hit, either from the construction of the index or the want of 

 a proper fall of light, the line of junction of the mercury and glass may 

 be taken. In that case, the tapping should never be omitted. Which- 

 ever mode of reading is once adopted should be stated, and always 

 adhered to. A piece of white paper placed behind the upper part of 

 the tube will generally enable any one to read off by the convexity of 

 the quicksilver. , In placing the index, notice whether it appears to 

 shift a little up and down as the eye is raised or depressed ; this is 



