CHAP. III. 



METHOD OF SUSPENSION. 



55 



The stand usually supplied by instrument makers (see Fig. 1) 

 is one of the worst that can be devised for use out of doors. It is 

 bad because the base (D E F) of the pyramid formed by the tripod 

 is not large enough in proportion to its height ; because the point 

 of suspension (A) of the barometer (B C) is much too low ; because 

 the legs (A D, A E, A F) of the tripod are fixed in a rigid position, 

 and cannot be set in or out to accommodate the irregularities of 

 the ground ; and upon snow it is nearly useless because these thin 

 metal legs sink in to or beyond the cistern (C) before a sufficient 

 foundation can be obtained. A moderate breeze puts the baro- 

 meter in movement (though it should be perfectly at rest for a 

 good observation) and a slight knock may make it oscillate from 

 G to H. If the cistern should swing as far as X the stand will 

 overturn, and the barometer almost certainly will be destroyed. 



I discarded the usual method of suspension, and hung the 

 barometers from the stands belonging to the theodolite (A J K L, 

 Fig. 2). The stability of these stands is infinitely greater than 

 that of the form usually employed ; their legs can be set in and 

 out to meet the requirements of each occasion ; and, when hung 

 in the manner shewn in the diagram, barometers have little tend- 

 ency to swing to and fro. No shock that is likely to occur will 



