it in a variety of positions; a different reading will be given for each 

 condition. 



50. Goldschmidfs aneroid. — The numerous levers and links in the 

 common aneroid are dispensed with in this form, and the minute 

 movements of the cell and spring are measured directly by means 

 of a micrometer screw. 



This is accomplished in several different ways by manufacturers, 

 a common form of instrument being shown in Figure IT, where 

 the parts have been separated for a better view. The plate B, with 

 its attached mechanisms, is secured in the bottom of the box A. The 

 micrometer screw S' works through the cover of the box. The 

 corrugated aneroid vacuum chamber M is held distended in the 

 usual manner by the steel spring E. A sharp knife-edge projection 

 a of a double-formed lever I rests upon a smooth polished spot near 

 the outer end of the spring. This spot is sometimes a bit of glass or 

 agate. The lever I is pivoted delicately upon an axis at r and is 

 formed of two parts joined near the axis. The upper piece of this 

 lever is a very delicate steel spring, with a flat polished surface at c, 

 which by the springiness of the arm presses against the point of 

 the micrometer screw S'. At the ends the spring and lever are 

 formed with little flat surfaces, each having a fine line engraved 

 across the middle. This construction is not clearly seen in the 

 drawing. To observe the air pressure the aneroid must be " set " 

 by bringing the above-mentioned lines into coincidence. For this 

 purpose, and at the same time to measure the movement necessary to 

 bring about such a coincidence, the finely cut micrometer screw S' 

 is provided. The large head S, having a scale of graduations en- 

 graved upon its outer rim, being turned, the point of the screw 

 presses against the spring at c, and deflects it so that the lines upon 

 the ends of the spring and lever may be placed in exact coincidence. 

 To facilitate making this adjustment accurately, a small magnifying 

 glass L is generally provided. A small scale is opposite the ends of 

 the lever I when the mechanisms are in their normal position and 

 indicates the whole number of turns made by the screw, or, what is 

 the same thing and more convenient, shows the pressure correspond- 

 ing to the successive positions of the screw. The fractions of a turn 

 are indicated accurately by the graduations on the head of the screw. 



51. Temperature e-ffects. — The Goldschmidt aneroid is not compen- 

 sated for temperature, but is generally accompanied by a table of 

 corrections therefor, the temperature being indicated by a small 

 thermometer, the scale of which, in the aneroid shown, is visible 

 through an opening at T. 



52. Reading, how' made. — Aneroids of this pattern are read by first 

 turning the micrometer screw until the lines upon the spring and 

 lever come into exact coincidence. The reading on the scale is 

 noted, and to this is added the part taken from the graduations on 

 the head. 



53. Consult paragraphs 104 to 106 for information respecting the 

 use of the aneroid in determining elevations. 



54. Ho'm adjusted to standard pressures. — The aneroid barometer 

 no matter how perfectly constructed, does not indicate any particular 

 pressure until by careful comparison with a standard barometer its 

 index is adjusted to give as nearly as possible the same reading as 



311681° — 11 4 



