1837.1 



Se If -Registering Barometer, 



67 



the principle of specific gravity. This latter mode, which has the very 

 great advantage of not losing power, consists in so proportioning 

 the thickness of tube or size of cup in the cistern to the quantity of 

 mercury added to the column by atmospheric pressure, that the tube 

 can be moved by a small increase of column to any length required. 

 In the case of a tube of uniform bore throughout (where no additional 

 weight of mercury is added by moving the tube downwards), it is obvious 

 that the thinner the lower end of the tube is, a proportionally greater 

 length of it must dip into the mercury in the cistern, before a volume 

 of mercury is displaced equal to what has entered the tube. But, in 

 the case of the tube with expanded bore at top, it is evident that after 

 atmospheric pressure has produced its full effect in increasing the 

 weight of the mercurial column, further weight is added merely by the 

 continued depression of the tube — because this depression magnifies, 

 according to the length of it, an equal extent of the small area of nar- 

 row portion of the mercurial column to the large area of expanded 

 top. This will appear evident on referring to Mr. Taylor's second 

 diagram, in which if c c represented the bottom of the expanded por- 

 tion before the tube be moved down, the upper portion of the narrow 

 column d will after the tube descends be magnified to c c. 



With respect to the expanded tube, therefore, when it is wished to 

 magnify the indications, the increased weight of mercury occasioned 

 by a given atmospherical pressure, must be added to the weight occa- 

 sioned by the descent of the tube to the extent of the enlarged indica- 

 tion wanted, and the area of the float made of such size as the product 

 of it into the length of the enlarged indication will equal the sum of 

 the bulks of the two weights of mercury just named. Suppose it is 

 desired to magnify the indications five times — expanded upper portion 

 of tube being two inches square, small bore of tube one-tenth of an inch 

 square. Here, for T V of an inch of rise of mercurial column, the tube 

 must descend \ inch; taking the cubic inch of mercury to weigh 3434, 



Then one cubic inch (= £ of 2 cubic inches) = 3434 



From which deduct weight of mercury already 

 in bore — _u inch area § inch long. 17-17 



Weight of mercury added by \ inch depression 



of tube. ■= 3416.83 



of one cubic inch (== T \of two cubic inches) 

 added by atmospheric pressure. 686.8 



Sum of weight of mercury added by pressure and 



depression = 4103.G3 grains., 



rri 1(in \ cub. inch raer. . . . area sq. inch . 4103.6% 9 on ..... 



Alien » 1717 .4103.63:: l . ■ 1717 - = *.oJ squaie 



inches the area of the float, A correction is of course required for the 

 thickness and specific gravity of the substance of the cup float. In an 



