Meteorological Observations. 141 



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-bubbles, 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 objec- 

 tionable amount above the quicksilver, it will not tap sharp against the upper 

 end of the tube when the barometer is quickly inclined from a vertical 

 posilion, 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 all, 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 temper- 

 ature 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 col- 

 umn of quicksilver shake visibly, 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. Whichever mode of reading is 

 once adopted should be stated, and always adhered to. A piece of white pa- 

 per 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 de- 

 pressed ; this is called Parallax, and is a source of uncertainty to be avoided 

 by placing the eye in reading always on the exact level of the top of the mer- 

 curial column. 



Barometric observations require corrections of three kinds, and to render 

 them available and comparable with others, it is necessary that their amount 

 should be ascertained, and distinctly stated. The first is called the Zero 

 Correction. It includes sev^eral subordinate corrections arising from differ- 

 ent sources, such as that originating in a faulty placing of the scale of inches, 

 that due to the capillary depression of the Mercury in the glass-tube, and the 

 constant part (which at a fixed station is nearly the whole) of the depression 

 arising from the presence of air or vapour in the upper part of the tube. 



To determine the zero correction, the Barometer must be compared with 

 a standard instrument, such as that at the Royal Observatory for instance, 

 or some other which has been compared with it, or with some standard of 

 equal authority. Such comparison ought never to be omitted before for- 

 warding the Barometer to its place of destination, nor should any op{)ortu- 

 nity be neglected of comparing it, when fixed in its place, with a good por- 

 table Barometer. In making such comparisons, all that is necessary is to re- 

 cord the readings of both the instruments, after at least an hour's quiet ex- 



