die Temperature Corrections of Barometers. 411 



In order to fix the position of the top of the lines giving the 

 corrections 1, 2, 3, 4 &c. millim., all that we have to do is to 

 make B t =800, and calculate the temperature t, which would 

 make the above expression equal to 1, 2, 3, 4 &c. millim. 

 Thus, for the top of the line representing a correction of 

 4 millim., we have 



(/3-y).800.*=4. 



For all ordinary purposes we may make ft — 7 for mercury 

 and a glass scale = 0-000181 -0-000009 = 0*000172, hence 



0-000172x800" 



But since the thermometer is not graduated, we must multiply 

 the value of t so found by n to get the distance in millimetres 

 of the point 4 from the point 0. Similarly the tops of the 

 other lines may be found, but in general it will be found 

 sufficient to calculate the greatest correction only, and then to 

 divide the distance between it and into the required number 

 of equal parts. In the same way the corrections correspond- 

 ing to a barometric height of 700 millims. (the lower ends of 

 the lines) may be obtained by making B^=700. Then, since 

 the correction is proportional to the height of the barometer, 

 straight lines joining 1 and 1, 2 and 2, and so on will repre- 

 sent the corrections between 700 and 800 millims. The spaces 

 between and 1, &c. may also be subdivided into halves or 

 tenths if necessary. 



The scale is either drawn or engraved on paper, a plate- 

 glass mirror, or other convenient material, and then mounted 

 on a bridge in front of the stem of the barometer and behind 

 the correcting instrument. The final adjustment is made by 

 moving the correcting thermometer into the proper position 

 before clamping it tightly to the stem of the barometer. A 

 piece of wood or other soft material interposed between the 

 stems of the barometer and thermometer prevents any risk of 

 breakage on screwing up. In adjusting the correcting 

 instrument or thermometer, care must be taken that, firstly, 

 the zero point of the thermometer is precisely in front of the 

 line of zero correction, and, secondly, that the thread of the 

 mercury is truly horizontal, and that its position between the 

 700 and 800 millim. fines of the correcting scale corresponds 

 as nearly as possible with the actual height of the barometer 

 at the time. 



For convenience, the completed barometer should be sus- 

 pended on a vertical wall with a good light falling on it. In 



