Xjlfj Instructions Jor Making and Registering 



posure, 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 man- 

 ner, 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 trans- 

 ported 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 observa- 

 tions, falls wholly on the determination of the heights of the station of obser- 

 vation above the mean level of the sea, and effects little, if at all, any conclu- 

 sions 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 ^nd 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 considerably inclined, 

 and inverted, and over all rough roads should be carried in the hand, to break 

 the shocks to which it would otherwise be exposed. If 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 de- 

 struction. Strapped obliquely across the shoulder of a horseman, however, 

 it travels securely and well, and with common care in this mode of transport, 

 its zero runs no risk of change. 



Tbe next correction, and the most important of all, is that due to the tem- 

 perature of the IMercury in the Barometer tube at the time of observation. 

 To obtain this, every Barometer requires to have attached to, or fixed very 

 near it, a Thermometer, called the attached Thermometer, which must be 

 read and registered at each observation of the Barometer. It is preferable 

 in practice to read off this Thermometer jf?rs/, to avoid the error arising from 

 breathing on, or standing long near it, while reading the Barometer itself. 

 The zero of this Thermometer should be ascertained by comparison with a 

 standard at the temperature of about 60° Fahr. 



The third correction applicable to barometric observations arises from 

 change of level of the mercurial surface in the cistern, owing to the transfer 

 of a portion of its contents to or from the tube. In Barometers with small 

 cisterns, and where the lower level cannot be adjusted at each observation, 

 its amount may be large, and its effect being always to make the apparent 

 fluctuation less than the real, in a Jlxed proportion, it ought, if possible, to be 

 ascertained. The data necessary to be known are — first, the internal and 

 external diameters of the tube — secondly, that of the cistern containing the 

 mercury, at the surface, where the tube plunges into it. These particulars, 

 as they must be known to the maker, ought to be inquired of him, and indeed 

 ought to be engraved conspicuously on some part of the instrument. 



Although all these corrections are necessary for the strict reduction of re- 

 gistered observations, they ought not to be applied to individual observations 

 previous to registry. It is sufficient to know them. Their effect is in all 

 cases easily and safely applicable to mean results, and to the conclusions there- 



