148 PNEUMATICS. 



hermetically sealed at the top, and the lower end 

 is open, and placed in a small basin of mercury. 

 The scale at the upper part, which is also shown 

 more at large at Fig. 3., indicates the parts of 

 inches which the mercury rises or falls ; and a slider 

 of metal, which can be placed to the exact line of 

 the surface of the mercury, enables one to observe 

 how much it has varied since the last observation. 



In this construction, as the space through which 

 the upper surface of the mercury in the tube has 

 to rise or fall, does not exceed 4 inches at most, 

 that is, from about 27 inches to about 31 inches 

 above the mercury in the basin, several contriv- 

 ances have been used to increase the scale, and 

 thereby show more sensibly small changes. The 

 chief of these are as follow. 



The diagonal barometer, ABC (Fig. 4.), is a tube 

 sealed at C, immersed in mercury at A ; this tube 

 is perpendicular from A to B •, where the scale of 

 variation begins at B, it is bent, as C B. The part 

 B C proceeds to the highest limit in the scale of 

 variation, I C ; consequently, while the mercury 

 rises from I to C in the common barometer, it will 

 move in this from B to C; and so the scale will 

 be, by this means, enlarged in the proportion of 

 B C to I C. 



However, this form being subject to a great de- 

 gree of friction, on account of the obliquity in the 

 part B C, which inclination makes the quicksilver 

 frequently divide into several parts, it requires the 

 trouble of filling tubes anew too often. 



The horizontal, or rectangular barometer, consists 

 of a tube, A C D F (Fig. 5.), sealed on the upper 

 end A, and bent to a right angle at D. The mer- 

 cury stands in both legs from E to C. Here, it is 

 evident, that in moving three inches from A to C, 



