436 



embraced by these changes, a correction could be readily applied. 

 Suppose, for instance, that a section of the cistern is one hundred 

 times that of the tube, or that their diameters are as 1 to 10, and 

 that the surface of the mercury in the cistern coincides with the 

 point from which the graduations commence when the mercury in 

 the tube stands at 30 inches. The correction would be one hun- 

 dredth part of the difference from 30 inches, and additive or sub- 

 tractive, according as this difference was below or above 30. 



It is usual, however, in the best barometers to bring the surface 

 of the mercury in the cistern to the point from which the gradua- 

 F lg .2S2. tions commence by means of a screw V, acting on a flexible piece 

 of leather which forms the bottom of the cistern. We are able to 

 tell when the desired coincidence is effected by means of a mark on 

 a piece of ivory floating on the mercury and sliding over a fixed 

 object having a corresponding mark. Sometimes the cistern is of 

 glass, and the point of commencement of the graduations is marked 

 upon it, or (which is much better) is indicated by the contact of 

 Fig. 231. a sharp ivory pin P, inserted in the cap, and descending into the 

 interior of the cistern. 



There are various kinds of portable barometers constructed for 

 the purpose of measuring the heights of mountains. The latest and 

 most convenient is represented in figure 232. It is of the syphon 

 form, and was invented by Gay-Lussac. The barometer being filled, 

 the extremity of the shorter branch Y is hermetically sealed. In 

 this state the barometer is inaccessible to the external air, and con- 

 sequently is incapable of indicating the changes of pressure in the 

 atmosphere; but to open the communication, we draw out, by means 

 of a blow-pipe, a small portion of the glass near the middle of the 

 shorter branch, on the inside, and form a fine capillary tube, which is 

 sufficient to admit the air but does not allow the mercury to escape, 

 on account of the force with which it repels it in virtue of its capil- 

 lary action. The difference of level between the two extremities 

 S, A', of the column being observed, it is reversed, and a part of the 

 mercury enters the longer branch CX, and fills it, the rest falls into 

 the shorter branch CF, but cannot escape for the reason above 

 mentioned. It may then be carried in this position, being always 

 open to the air, but not to the mercury. 



This barometer may be enclosed in a cane and transported with 

 great ease and safety. A small thermometer is appended, as in 

 other cases, for the purpose of measuring the temperature of the 

 mercury. By contracting the tube near the two ends we prevent 

 all danger of its breaking by any sudden motion in the column of 

 mercury. 



