592 SEC. 14. METEOROLOGY. 



be calculated beforehand by means of precise formulae from constants that are 

 easily determined with great exactness. The apparatus has, therefore, nothing 

 of the nature of an interpolating instrument, a character belonging at present 

 to all registering mechanisms, but must be viewed as a provision instrument. 

 Checking observations, daily required by all self-registering instruments, be- 

 come superfluous with this apparatus. 



(2.) The registering instruments can be put up in a dry place, protected 

 against changes of temperature. 



(3.) All parts of the apparatus are of metal and glass, wood is excluded. 

 The instrument is, therefore, more durable. 



(4.) The construction is extremely simple ; any disorder is easily repaired. 

 The purely mechanical motors are free from the disturbances to which 

 electro-magnetic motors are so frequently subject. If moderately well executed, 

 and tolerably well handled, the instrument will, practically, never refuse to 

 work. 



(5.) The registering of the temperature of the instrument might prove 

 useful in other observations, since it gives the temperature of a closed space. 



(6.) Not the least of the advantages is the cheapness of the instrument. 



The theory of this apparatus is given in full in Volume XI. of Carl's 

 " Repertorium fur Experimentalphysik." 



2891. Kreils' Barograph, formerly in use at the Kew Obser- 

 vatory, for registering the movement of the barometer. 



Kew Committee of the Royal Society, Kew Observatory. 



An instrument employed at the Kew Observatory in 1845, for the purpose 

 of registering automatically the height of the barometer. It consists of a 

 syphon barometer, having a float resting upon the surface of the mercury, in 

 the open end of the tube. Immediately above the tube a lever is fixed 

 horizontally, and a cord, wrapped round a sector on the short arm, passes 

 down and is attached to the float. The other end of the lever carries an 

 ordinary pencil, which, being struck every five minutes by a hammer moved 

 by a clock, makes a dot upon a sheet of paper fixed to a frame drawn in front 

 of it by clockwork. 



2892. Ronalds' Photo-Bar ometrograph, for registering 

 photographically the changes in the height of the barometer, for- 

 merly erected at the Kew Observatory. 



Kew Committee of the Royal Society y Kew Observatory. 



An instrument for registering the variation in the height of the barometer 

 upon a daguerreotype plate; constructed in 1847 by Mr. Francis Ronalds, 

 afterwards erected at the Kew Observatory, and described by him in the 

 British Association Report for 1851. 



The light from an argand lamp, after passing through a condensing lens, 

 falls on a narrow slit cut in a metal plate attached. A barometer tube, the 

 mercury in which, by rising or falling, varies the length of the slit illuminated. 



An achromatic combination of lenses, by Voigtlander, throws an image of 

 the bright slit, magnified about 200 times, upon an aperture in the case, past 

 which a daguerreotype plate is moved slowly by clockwork, and so registers 

 the changes in the height of the barometer. 



The barometer itself, together with its cistern, which is of large area, is 

 suspended from an arrangement of levers and zinc rods, on the principle 

 of the gridiron pendulum, in such a manner as to render the indications 

 unaffected by fluctuations of temperature. 



An improved form of this instrument, in which the photographic image is 

 impressed upon paper, is now in use at Kew, and at many other observatories. 



