Meteorological Observations. SQ* 



position so as lo make the merciuy rise above its level, nearly 

 to tlie top, and thsu ^entV^ jpiked lengthways and backwards. 

 If the blov/ is puPi'y 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 liiiht near a window, 

 but not exposed to sunshine, in a retired apartment, little 

 liable to sudden changes of temperature or to drafts of wind. 

 A ijust 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 coluipn of quicksilver shahc visibly, as the 

 mercury is apt to adhere to the glass and give erroneous read- 

 ings. 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 paper placed behind the upper part of 

 the tube will generally enable any one to read off' by the con- 

 vexity of the quicksi.ver. In placing the index notice whether 

 it appears to shift a little up and down as the eye is raised or 

 depressed. This is called Parallax, and is a source of un- 

 certainty to be avoided by placing the eye in reading alway-i 

 on the (xnct hvl of the top of the mercurial 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 tirst is called the Zero Correction. It includes 

 several subordinate corrections arising from different 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 pnrt (which at a fixed station 

 is nearly the whole) of the depression arising- from the ])reseuce 

 of air or vapour in the upper part of the tube. 



To determine the zero correction, (he Barometer must be 

 compared with a standard instrument, such as that at I ho I'oyal 

 Observatory for instance, or sr)me other which has been com- 

 pared with it, or with some standard of equal authority. Such 

 comparison ouf;ht never to be omitted before forwarding; the 

 Barometer to its place of destination, nor should any oppor- 

 tunity be neglected of comparing it, when fixed in its place, 

 ■with a good portable Barometer. In makinjr such com)iarison8, 

 all that is necessary is to record the readings of both the 



