47 



striiment itself acts as a plumb line and takes a vertical position witli 

 sufficient accuracy. It is desirable, however, for convenience in set- 

 ting the barometer, as well as to insure the permanent verticality 

 of the instrument, to steady it in supports which are first adjusted, 

 once for all, so that the barometer is accurately vertical, as deter- 

 mined by a plumb line applied alternately at the front and one side. 



91. Improved haromefer box. — Tlie standard pattern of barometer 

 box now in use by the Weather Bureau is shown in Figure 29. 



This box must be securely attached to the wall in a location afford- 

 ing good light and not subject to sudden changes of temperature. 

 In many cases it will first be necessary to fasten to the wall hard- 

 wood strips, to which the top and bottom of the barometer box can 

 be secured by screws passing through metal plates provided for this 

 purpose. Place the screw for holding the top of the box in the 

 center of the top strip and suspend the box 

 thereon. When the box is set about vertical 

 (as determined by a plumb line) secure it 

 firmly at the bottom by a screw passing 

 through the metal plate into the wooden 

 strip. 



The hooks in the top of the barometer 

 box will, upon examination, be found to be 

 adjustable in their positions. Place them 

 in their central positions and hang thereon 

 the " station " and " extra " barometers. 

 Next, find the positions at the bottom for 

 attaching the ring-shaped guides, one of 

 which is shown in Figure 30. These rings 

 should be slipped over the lower end of the 

 barometer, and placed about the mid height 

 of the cylindrical part of the cistern. Mark 

 the screw holes and fasten the rings to the 

 back of the box. The barometer when at 

 rest should hang freely within this ring, 

 and may be adjusted to do so by the hook at 

 the top. When the rings are fitted with 

 centering screws these should be carefully 

 screwed up until the barometer cistern is very gently clamped and 

 held steadily, but not in the least deflected from its vertical position 

 when free. 



Barometers that swing slightly free witliin the ring support 

 should, in setting, be steadied against one side of the ring, as shown 

 in Figure 30. The following caution must, however, be observed. 



92. Caution against error. — In adjusting the barometer for reading 

 it is very important that it be steadied against the ring in the proper 

 manner, otherwise an appreciable error is introduced, because of 

 imperfect verticality. 



If I, Figure 30, is the position of the ivory point, then steady the 

 cistern against the ring at A or at B, but never at C or D, or at other 

 points. A and B should ahoays he in a line exactly at right angles to 

 a line through the ivory point and the center of the ciMern. 



A little thought will show the necessity for this. If, for illustra- 

 tion, we imagine the ivory point at one side and just in contact with 



Figure 30. — Barometer cistern 

 and ring support 



