202 



Instructions for Making and Registering 



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 mercurial 

 column. f 



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 several subordinate corrections 

 arising from different sources, such as that originating in a faulty plac- 

 ing 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 forwarding the Barometer to its place of destination, nor should 

 any opportunity be neglected of comparing it, when fixed in its place, 

 with a good portable Barometer. In making such comparisons, all 

 that is necessary is to record the readings of both the instruments, after 

 at least an hour's quiet exposure, side by side, that they may have the 

 same temperature. If compared by two observers, each should read off 

 his own Barometer in his usual manner, and each should take a mean 

 of several readings, then each should verify < the other's results. By 

 this means the zero of one standard may be transported over all the 

 world, and that of all others compared with it ascertained. 



The amount of the zero correction is often very large, as two or three 

 tenths of an inch, but its influence on the mean results of recorded ob- 

 servations, falls wholly on the determination of the heights of the station 

 of observation above the mean level of the sea, and effects little, if at 

 all, any conclusions of a meteorological nature which may be deduced 

 from them. Hence, if proper care be taken to preserve a Barometer, 

 once set up, immoveable, a long and regular series of observation with 

 it has a value independent of any knowledge of this element, and it is 

 fortunate that this is the case, as the zero correction is one extremely 

 difficult to determine exactly a priori. 



In transporting a compared Barometer to its place of destination, 

 great care is necessary. It should always be carried upright, or con- 

 siderably inclined, inverted > and over all rough roads should be 

 carried in the hand, to break the shocks to which it would otherwise be 

 exposed. Ef strapped horizontally under the roof of a colonial waggon, 

 or tied upright against the wood-work, with its head resting on the floor, 

 there is not a chance of its escaping destruction. Stiapped obliquely 

 across the shoulder of a horseman, however, it travels securely and 



