528 Instructions for Mak'utg and Registering 



instriimeiits, aftir at least an hour's quiet exposure, side by side, 

 tluit thcv may have ihe same tcmpeiatuie. If compaied by 

 two observers, each siioiihl read otT his own Barometer in h.s 

 usual maniicr, and each should take a mean of several readings, 

 tlien eacli should verify the other's result. By this means the 

 zero of one sla;idard may be transported over all the world, 

 and that of all others compared vrith it ascertained. 



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

 or three-tcnlhs of an inch, but its inrluenco on the mean results 

 of recorded ubscrvatior.s, falls wholly on the determination of 

 the heijrhts of tliC station of observation above the mean level 

 of the sea, and afl'ects little, if at all, any conclusions of a 

 meteorological nature vvhicli may bo dediiced from them. 

 Hence, if proper care be taken to preserve a Barometer, oi:c3 

 set up, immoveable, a long and regular series of observations 

 with it has a value indopcndont of auv knowledge of this 

 element, and it is fortunate that tliis is the case, as the zero 

 correction is one extremely difficult to determine exactly 

 a prion. 



In transporting a compared Barometer to its place of desti- 

 nation, great care is necessary. It should always be carried 

 j<p'ifjli/, or considerably inclined, asid intertcd, and over all 

 rough roads should be carried in the hand, to break the shocks 

 to which it would otherwise he exposed. If strapped horiz'ni- 

 tally under tlic roof of a colonial wagon, or tied upright ngainst 

 the wood-work, with its head resting on the floor, there is not 

 a chance of its escaping destruction. 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. 



The next correction, and the most important of all, is that 

 due to the temperature of the Mercury in the Barometer tube 

 at the time of observation. To obiaiu this, every Barometer 

 requires to have attached to, or fixed very near it, a Thermo- 

 meter, called the attached Tlieinionieter, which must be read 

 and rofjistered at each observation of the Barometer. It is 

 preferable in practice to read off this Thermometer first, to 

 a\oid 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 {>urfacc 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 cacli observation, its 

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



