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EXPLANATION OF PLATE XIII. 



Fig. 1. Represents a section of an iron cistern for a barometer. 



d. An orifice for the introduction of the mercury, afterwards shut up by 



means of a screw. 



e. The air-duct, having a screw formed on its outer surface. 



f. An air-tight stop-cock, having a female screw, by which it is attached to 



the air -duct. 

 g. A small orifice in the side of the stop-cock, to serve as a passage for the 



air, and so as to exclude dust. 

 h. A lever connected with a timepiece, by means of which the stop-cock was 



shut, and the communication of the air with the mercury cut off at any 



given instant. 



Fig. 2. Represents an instrument for determining the height of the barometer each 

 hour in succession, by three observations in the course of the day. 



It consists of a hollow column of wood about 44 inches in diameter, and 84> 

 inches in height ; the base being about 2 inches in height, and 6J in dia- 

 meter. 



It will be found convenient to have the column divided longitudinally into 

 two equal parts, and united by means of hinges. 



At equal distances, on the circumference of the column, are arranged seven 

 barometers ; their cisterns being inclosed in the base, and so placed that 

 their stop-cocks shall form a circle in the interior. By means of a hori- 

 zontal wheel nearly on a level with the stop-cocks, and which, from its con- 

 nection with a time-piece, revolves once in seven hours. One of the stop- 

 cocks are shut each hour in succession. The adaptation of the mechanism 

 is so free of all complexity, as to render a more particular description un- 

 necessary. 



Fig. 3. Represents the seven barometers arranged on a flat surface. 



a. This circle points out the situation of a vertical wheel which revolves once 

 in seven hours. Levers are connected with each of the stop-cocks ; and 

 their central extremities, being placed at equal distances, and forming a 

 semicircle around the circumference of the upper half of the wheel, 

 one of the stop-cocks are shut each hour in succession. 

 A spiral or other spring for turning the stop-cocks, and operated upon by 

 the levers by means of a catch, both simplifies the operation, and secures 

 accuracy to any given instant. 



