48 



the mercurial surface and the barometer vertical, it is very clear that 

 if we swing the barometer out of vertical a little either to one side 

 or the other, the ivory point will either dip into the mercury a little 

 or rise above the surface, whereas, if the barometer be deflected back- 

 ward or forward, there will be little or no per- 

 ceptible change in the level of the mercury at the 

 ivory point. 



93. Old-style haroineter hoxes. — The simplest form 

 of box used by this bureau is shown in an im- 

 proved form in Figure 31. The top end of the box 

 is made of thick material, which is recessed to re- 

 ceive the top portion of the barometer when sus- 

 pended upon the long hook H screwed into the top. 

 For reading, the barometer is drawn out upon the 

 hook, as shown in the figure, and returned to the box 

 after the observation. The lid of the box is fitted 

 to close under the hook so that the whole barometer 

 is thoroughly encased and well protected. Except 

 that the barometer must be shifted out of and into 

 the box every time readings are made, and the fur- 

 ther disadvantage that the cistern necessarily swings 

 free, this style of box answers the purpose in a very 

 satisfactory manner, 



94. The barometer should be carefully lifted along 

 the hook and not made to slide roughly or permitted 

 to knock against the guides in the box. 



95. When boxes such as described in the preced- 

 ing paragraphs are not furnished the barometer may 

 be suspended from almost any suitable hook securely 

 fastened to the wall in such location as will satisfy, 

 as far as possible, the conditions of paragraph 89. 



96. Mai^ne Ixiroineter hox. — This has already been 

 described in previous paragraphs 26 to 31. We need 

 only add that in attaching this box to the wall the 

 height must be regulated so that the scale comes at 

 the level of the eye when the hinged bracket is 

 lowered with the barometer in the reading position. 

 The barometer will be several inches too high to 

 read conveniently when folded into the box. 



9T. How to set and ohserve the harometer. — ^Hav- 

 ing in mind the various sources of error affecting 

 barometers, and other peculiarities of the instru- 

 ment, we may next consider how best to secure accu- 

 The presence of the observer's body near the barometer 

 tends to increase its temperature. The scale and outer parts are 

 affected first, then the thermometer, and much more slowly the mer- 

 curial column. Generally, however, this effect is slight, as only a few 

 minutes are required in making a reading. It is best to read the 

 attached thermometer first. Next, if the barometer is freely sus- 

 pended, jostle the cistern a little, so that the mercurial surfaces may 

 be detached if they tend to cling to the glass walls. To " set " the 

 cistern of the barometer, the level of the mercury should be lowered 

 a little by turning the milled head O, Figure 3, and raised again until 

 it is just in contact with the ivory point. To make this adjustment 



riQUEB 31. — Old- 

 style-barometer 

 box. 



rate readings. 



