THE SMITHSONIAN INSTITUTION. 



253 



figures 2, 3, and 4. To tlie lower piece 

 j is fastened the flexible bag w, made of 

 kid leather, furnished in the middle 

 with a socket k, which rests on the end 

 of the adjusting-screw 0. These parts, 

 with the glass cylinder F, are clamped 

 to the fl.ange B by means of four long 

 screws P and the ring R ; on the ring 

 R screws the cap s, which covers the 

 lower parts of the cistern, and supports 

 at the end the adjusting-screw 0. G, 

 i, j and k, are of box-wood ; the other 

 parts of brass or German silver. The 

 screw serves to adjust the mercury 

 to the ivory point, and also, by raising 

 the bag, so as to completely fill the 

 cistern and tube with mercury, to put the instrument in condition for 

 transportation. 



In Fortin's barometer, and also Delcro's modification of it, a cement 

 is used to secure the mercury against leakage at the joints. This, 

 sooner or later, is sure to give way ; and tested under the extremes of 

 the thermometrical and hygrometrical range of this climate especially, 

 has made this defect more evident. This was removed by the substi- 

 tution of iron in the place of wood ; but it was soon found impractica- 

 ble, in this form of cistern, to prevent damage from rust. These ob- 

 jections led to the present plan of construction, which effectually 

 secures the joints without the use of any cement. The surfaces con- 

 cerned are all made of a true figure, and simply clamped together by 

 the screws, a very thin leather washer being interposed at the joints. 

 This would not be permanent, however, but for the especial care taken 

 in preparing the box-wood. The box- wood rings are all made from 

 the centres of the wood and concentric with its growth. They are 

 worked thin and then toughened, as well as made impervious to 

 moisture, by complete saturation with shellac. This is effected by 

 immersing them in a suitable solution in vacuo. The air being with- 

 drawn from the pores of the wood, is replaced by the lac. This, 

 however, with the after-drying or baking, requires care ; but when 

 properly done, the wood is rendered all but unchangeable. 



Another peculiarity consists in making the scale adjustable to cor- 

 rect for capillarity, so that the barometer may read exactly 

 with the adopted standard, without the application of any 

 correction ; and this, too, without destroying the charac- 

 ter of the barometer as an original and standard instru- 

 ment. Near the 30 inches line, figure G, is a line v, on the 

 main tube ; this last line is distant exactly thirty inches 

 from the tip of the ivory point ; therefore, when these lines 

 coincide, or make one line, the scale is in true measurement 

 position ; or the 30 mark is exactly thirty inchesfrom the 

 tip of the ivory point in the cistern. In this position, the 

 amount of correction due to capillarity being ascertained, 

 the scale is then moved that quantity and clamped firm. 

 The barometer will now give the readings corrected for ca- 



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Mg.$ 



