236 THE BAROMETER. 



and mix the paints himself. The following are the requisite in- 

 gredients and their proportions, 



White 



Parts. 



| White lead 



Dryers (litharge) 



Linseed oil 



^ Spirits of turpentine 

 (turps) 



Pts. 



Chinese red (or red 



lead) 

 i Dryers 



Lunseed oil 



Turps 



Pts. 



| Vegetable black 

 Dryers 

 ! 3 g Linseed oil 

 i a 6 Turps. 



The solid materials must be in a finely powdered state ; they are 

 thrown first in the vessel, in which the paint is made up, and the 

 liquids are poured in afterwards on the top. The whole' is then 

 well stirred until the mass is of uniform consistence. 



The so-called ' black -japan ' may also be used as a black varnish, 

 and a red varnish may be made by dissolving a bit of seaKng-wax in 

 spirits of wine. The brushes may be cleaned with oil of turpentine ; 

 or, if any varnish has been used, with a little spirits of wine. 



FIG. 163 (I real size). 



When the tube is to be removed, it must be- inclined very cau- 

 tiously, otherwise the mercury may strike so hard against the closed 

 end as to break it off. 



A barometer which is to be used not for a single 

 experiment, but for repeated accurate measurements 

 of the atmospheric pressure, must be specially con- 

 structed for the purpose. Barometers in which the 

 tube dips into a vessel containing mercury are called 

 cistern-barometers] in another class of barometers the 

 tube is bent upwards near the open end, as seen in fig. 

 164; these are called siphon-barometers. The siphon- 

 barometer is more convenient for transport when 

 observations are to be made at different places, and 

 the error arising from capillarity is excluded if both 

 branches of the tube are equally wide, for in that 

 case capillarity depresses both ends of the mercury 

 equally, and the length of the mercurial column, that 



