

METEOROLOGICAL INSTRUMENTS. 219 



Every barometer is provided with an " attached thermometer," 

 in order to show the temperature of the mercury at each reading. 

 This thermometer is fitted in various ways. 



The words "closed cisterns," which have been used above, 

 need some explanation. It is found that if the cistern be made 

 of wood, the pressure of the atmosphere will exert its action 

 through the pores of the wood on the level of the liquid in the 

 cistern. If the cistern be made of iron, a small hole is left, which 

 is closed with a leathern plug permeable to the air pressure, but 

 not so to the mercury. 



Self -recording instruments. Barometers are made self-recording 

 in various ways, either by photography, electricity, or by me- 

 chanical action. All such instruments are termed Barographs. 

 The first method gives a continuous record, and is employed at 

 Kew and Greenwich. 



In the Kew instrument, devised mainly by Sir F. Ronalds, the 

 record is produced by allowing light to pass over the column of a 

 cistern barometer, in this way photographing the space of the 

 Torricellian vacuum at the top of the tube. Ronalds' original 

 instrument is exhibited. 



In the Greenwich instrument, devised mainly by Mr. Charles 

 Brooke, the barometer is of the syphon form, and the record is 

 obtained by photographing the position of a float resting on the 

 surface of the mercury in the open limb. 



The second method affords an intermittent record, and has 

 been very frequently employed. The most usual process is to 

 cause a wire to descend at frequent intervals until it touches the 

 surface of the mercury in the open limb of a syphon barometer, 

 and causes a galvanic current to pass, thereby eventually pro- 

 ducing a mark on paper registering the reading. This method 

 was first proposed by Sir Charles Wheatstone, and has been 

 adopted, with modifications, by Dr. Theorell of Upsala, and Pro- 

 fessor Van Rysselberghe of Ostend. The last-named gentleman 

 exhibits his complete meteorograph, which records on the same 



