OF METEOKOLOGICAL INSTRUMENTS. 



A 



FIG. 20. 



Barograph and Thermograph, Self-registering (BECKLEY'S). These interesting 

 arrangements were designed by Mr. Beckley at the express desire of the 

 Kew Committee of the British Association for the Advancement of Science. 

 They faithfully and permanently record the varying changes in atmospheric 

 pressure, temperature, and moisture, by means of photography. And together 

 with the anemometer No. 82 are now in constant use at their observatory. 

 Where ordinary gas is at hand its light is employed, but, when otherwise, 

 a convenient form of lamp is arranged, in which colza or paraffine oil is 

 used, and although requiring rather more care, is even preferable to gas. 

 The knowledge of photographic manipulation in these arrangements is 

 easily attained, and the time and attention required for this purpose is reduced 

 to a minimum by using prepared paper and changing it every twenty-four 

 hours 



20. Barograph. The design of this instrument is to record the varying changes of 

 barometric pressure by means of photography, as shown on fig. 20. 68 



"When in use the upper part is enclosed in a mahogany box or cover, which is here removed 

 to show the various parts. Artificial light alone is employed, and is admitted only through the 

 verticle slit in the shield fc. a is the cylinder, or drum, on which the photographic paper is 

 placed, and is turned round once in twenty-four hours by means of the clock 6. Besides 

 turning the cylinder the clock also liberates the small shutter c, which then turns sharply 

 round once every two hours, thus stopping off the light for four minutes each time, leaving 

 white lines on the photographic curve which represent intervals of two nours each, g is the 

 burner or light ; / a condensing lens through which the rays pass over the top of the mercurial 

 column. On the light passing through the slit at Ic, from the lamp g, it passes through the 

 photographic lens Ti, and thence on to the cylinder a ; pis the barometer, the mercury of which 

 rises and falls immediately behind the shield Jc ; g is a screw by which the barometer is lowered 



