L. CASELLA'S CATALOGUE 



FIG. 22. 



or raised when adjustment is required ; n n are two zinc rods firmly screwed at their lower end s 

 to the verticle slab A, and at their upper end a plate and socket are fixed which, carries th e 

 short pendant glass rod j, together with small rollers by which its movements are kept free on 

 the slab A; the lower point presses on the horizontal glass lever d, near the fulcrum I ; by 

 this arrangement the expansion and contraction of the zinc rods from varying temperature are 

 so expanded as just to compensate for the thermometric changes in the mercurial column, an 

 undulating line being thus formed which is always the zero line of the curve ; r is a glass tab e 

 of the same internal diameter as that of the barometer, and is half filled with mercury in 

 which the bulb of a standard thermometer is immersed. 



22. Thermograph. This instrument is designed to show changes of atmospheric 

 temperature and moisture, by means of photography ; and when in use is 

 enclosed in a mahogany box, for the exclusion of light in the same way as 

 the barograph; in this case, however, the artificial light to the paper 

 is only admitted through an air-speck in each thermometer, separating the 

 mercury in the same manner as that arranged by L. CASELLA to detach the 

 index in his maximum registering thermometer. An ingenious arrangement 

 supports the thermometer bulbs in the open air, they project about one foot 

 from the wall, upon the edge of which the slab rests. The general arrange- 

 ment being as in fig. 22 . .' *.- . 82 



m. Wet bulb thermometer. I. Atmospheric thermometer, g. The screw for adjusting 

 the thermometers to the height required, a a. Artificial lights. / i. Condensers to throw 

 the light on the mirrors fc n. kn. The mirrors passing light through the air- speck in each 

 thermometer, o o. The slits through which the light passes from the mirrors fc n. e e. The 

 lens throwing an image of the air-speck of each thermometer on to the cylinder c on 

 which the photographic paper is placed, d. The clock which turns the cylinder c once 

 round in forty-eight hours, b. The shutter which cuts off the light from the prepared 

 paper for four minutes, every two hours, and thus leaves a white line in the photographic 

 indication. 



