PNEUMATICS. 14*9 



it will move through so many times three inches in 

 the small leg D F, as the bore of D F is less than 

 the bore of A C j whence the motion of the mer- 

 cury at E must be extremely sensible. This form 

 is liable to the same exceptions as Fig. 4. ; and, 

 besides a great degree of friction, and the frequent 

 breaking off of the mercury in the leg E, the part 

 D F being a very small bore, the free motion of 

 the mercury therein must be impeded by the at- 

 traction of cohesion. 



The wheel barometer. — A (Fig. 6.) represents 

 the quicksilver in a glass tube, having a large round 

 head or ball, and turned up at bottom B ; upon 

 the surface of the mercury, in the recurved leg, 

 there is placed a short glass tube loaded with mer- 

 cury, with a string going over a pulley, and ba- 

 lanced by another weight hanging freely in the 

 air. As the surface at A is very large, and that at 

 B very small, the motion of the quicksilver, and 

 consequently of the ball A, will at bottom be 

 very considerable ; but as the weight moves up 

 and down, it turns the pulley, and that a hand or 

 index j and, by the divisions of a large graduated 

 circle, the minutest variations of the air are plainly 

 shown, if the instrument be accurately made, 

 and the friction of the several parts be incon- 

 siderable. 



There is also a barometer, contrived so as not to 

 be affected by the motion of a ship, called the 

 marine barometer. 



Also a portable barometer, for moving from place 

 to place without injury, and for measuring the 

 heights of mountains, by observing the difference 

 of the altitude of the mercury at the bottom and 

 top of the mountain. 



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