THE BAKOMETEE. 



Fig. 4. 



mercury has been invariably retained as the only practicable 

 fluid for barometers. 



Several expedients, however, have been adopted in barometers- 

 used for common domestic purposes to render their indications 

 more sensible. Although these are inapplicable in barometers 

 used for scientific purposes, yet, as they are frequently adopted 

 in domestic barometers, it may be useful here to notice them. 

 11. A form of barometer, called the diagonal barometer, is 



represented in fig. 3. In 

 this the upper end of the 



jig. 3. tube is bent, so that the 



scale, instead of being 

 limited to the length c D,. 

 is extended over tha 

 greater length c B. 



12. A form of barometer, 

 called the wheel barome- 

 ter, is represented in fig. 4. 

 In this, the tube, instead of 

 having a cistern, is con- 

 tinued of the same diam- 

 eter, having its lower end 

 bent upwards at B c. A 

 float is placed upon the 

 mercury at F, which rises 

 and falls with it. The 

 change of altitude of the 

 level F corresponds with 

 that of E, and the differ- 

 ence between the two 

 levels E and F is the height of the barometric column. The 

 changes of this height are always double the change of level of 

 the surface E F. The float F is connected by a string with a 

 wheel, H, which carries an index that plays upon a graduated 

 dial plate, G. In this manner the magnitude of the graduated 

 scale may be made to bear any proportion, however great, to the 

 change of level of the mercury at E, so that the smallest change 

 of the barometric column will produce a considerable motion of 

 the index. 



13. One of the most common forms of this instrument is repre- 

 sented in fig. 5. The lower part of the tube being bent into the 

 form of a siphon, like the wheel barometer, is inserted in the 

 lower part of an iron stop-cock, the upper part of which is 

 inserted in a small globular cistern containing the mercury. 

 When the stop-cock is open the column in the tube is subject 

 182 



A 



