128 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



3. Before a storm or extraordinary high wind, the sub- 

 stance will be partly at the top, and will appear in form of 

 a large leaf ; and the liquid will be very heavy, and in a fer- 

 mentation. This will give twenty-four hours' notice before 

 the weather changes. 



4. In winter time, generally the substance will lie rather 

 higher, particularly in snowy weather or white frost ; the 

 composition will be very white, with white spots in motion. 



5. In summer time, the weather being very hot and fine, 

 the substance will be quite low. Price, $1.25. 



o 



40 



Fig. 614. Differential Thermometer. (Fig. 



614.) This instrument consists of 

 a long glass tube, twice bent at 

 right angles, having a bulb at each 

 extremity. The tube contains a 

 quantity of sulphuric acid, tinged 

 with carmine. The instrument is 

 furnished with a scale of 100 equal 

 parts, and is fixed upon a wooden 

 support. Both the bulbs of the 

 instrument being exposed to the 

 same temperature, it is not in the 

 least affected ; but as soon as one 

 of the bulbs is exposed to a higher 

 temperature than the other, the 

 difference between them is deli- 

 cately shown by the falling of 

 the colored fluid below the bulb, 

 which is most heated. This in- 

 strument not being affected by the variations of atmo- 

 spherical pressure, nor by fluctuations of temperature in the 

 atmosphere, it is admirably fitted for experiments on ra- 

 diant heat. Price, $1.50. 



Howard's Differential Thermometer. (Fig. 615, next 

 page.) This instrument is a modification of the last, and 

 considered by some as more convenient, the difference con- 

 sisting in the balls being placed the one above the other. 



Price, $1.50. 



