METEOROLOGICAL INSTRUMENTS. 127 



alteration of level amounting to one inch in each tube, or 

 two inches in the height of the barometric column ; and as 

 the dial plate may be from twenty to thirty-six inches in 

 circumference, five to nine inches on the graduated plate 

 corresponds to one inch of the column, which is divided into 

 one hundred parts, each of which is distinctly perceptible. 



The frames are usually of mahogany, the best ones of 

 rosewood, highly polished ; the dial-plate is from six to 

 twelve inches in diameter, and elegantly engraved and 

 silvered, and is covered by a brass ring containing a convex 

 circular glass ; also a thermometer capable of being de- 

 tached, having a separate frame sunk into the body of the 

 frame of the instrument, and covered with a glass; the 

 thermometer tube having a long cylinder, instead of a round 

 ball, for containing the mercury, thus exposing a greater 

 surface to be acted on by the atmosphere. In the back of 

 the frame of the instrument is a cavity covered by a hinged 

 door, within which the tube and fixtures are enclosed. 



There is usually an index of polished brass, working on 

 the dial plate of the best instruments, movable by a key 

 below the dial, and serving to indicate the position of the 

 hand, when last observed. The mode of fitting up this in- 

 strument gives it an elegant appearance as a piece 

 of furniture. F ' l s- 613. 



Price of 8-inch, in mahogany frame, $12.00. 

 10-inch, " " $15.00. 



" 8-inch, best rosewood, $15.00. 



10-inch, " " $18.00. 



12-inch, " " $25.00. 



Prognosticator, or Storm Glass. (Fig. 613.) 

 A new curious instrument, formed of different com- 

 positions, which will indicate the weather ; particu- 

 larly high wind, storm, or tempest It will be 

 preferable by sea and land, being portable, and 

 will be found to be very exact and useful. 



Rules to be observed. 1. If the weather is to be 

 fine, the substance of the composition will remain en- 

 tirely at the bottom, and the liquid will be very clear. 



2. Previous to changeable weather for rain, the 

 substance will rise gradually, and the liquid will 

 be very clear, with the appearance of a small star 

 in motion. 



