the Temperature Corrections of Barometers. 409 



The precision of the complete barometer is limited not so 

 much by the accuracy of the temperature correction, as this 

 can easily be made to read correctly to 001 millim., but by 

 the precision with which the barometer can be set and by the 

 accuracy of the graduations. As has already been mentioned, 

 the combined error in setting the barometer and reading the 

 scale at the top should not exceed 0*1 millim. Of course all 

 danger from the above source is removed if the common 

 syphon form of barometer is employed; but as Collie's 

 modification presents other advantages which are clearly set 

 forth in his paper * , his form of barometer is to be preferred, 

 especially as it is only necessary for the purposes of the 

 correcting instrument that the above relation between n and 

 w/2 should hold good within a millimetre or two. 



In designing the correcting instrument use has been made 

 of the fact that the variation in position of a point on or 

 attached to the stem of the barometer is proportional to the 

 variation of the height of the barometer. The point is repre- 

 sented by a horizontal thread of mercury H (figs. 1 and 2), 

 contained in an ungraduated thermometer which is firmly 

 attached to the stem of the barometer in a horizontal position. 

 Behind the horizontal thread of mercury is fixed a scale or 

 small plate of curves K, in such a position and drawn in such 

 a manner that the position of the horizontal thread of mercury 

 (the ordinates) indicates approximately the height of the 

 barometer. The correcting instrument is shown on a larger 

 scale in fig. 2, The distance from the top to the bottom of 

 the plate of curves, i. e. from L to M, is actually 50 millims., 

 but this, from what we have already said, represents altogether 

 100 millims., the position of L corresponding to a barometric 

 height of 800 millims., whilst M corresponds to 700 millims. 

 The position of the horizontal thread of mercury at the time 

 of setting the barometer thus corresponds approximately to 

 the actual height of the barometer. A series of lines 1, 2, 3, 4, 

 &c. millim. are drawn or engraved on the plate K, so that the 

 position of the meniscus of the horizontal thread of mercury 

 gives the temperature correction directly in millimetres. The 

 method of drawing these lines requires some explanation. It 

 is desired to reduce the height of the barometer to 0° C. 

 Obviously, then, the line of zero correction must lie 

 immediately behind the point corresponding to 0° C. on the 

 horizontal thermometer, and it must furthermore be vertical, 

 as no matter what the height of the barometer may be the 

 correction at 0° C. must always remain zero. 



* Collie, loc. cit. 



