24 



the standard. This adjustment is made by means of the screw, which 

 in nearly all aneroids is seen just within a small hole in the back of 

 the case. The graduations of the dial must of course be such as to 

 show changes of pressure on a scale of millimeters or inches. 



The Goldschmidt aneroid is similarly adjusted in a variety of 

 ways, of which a common one is to shift the zero or index line at 

 which the reading of the micrometer screw is made. 



55. Errors and defects of mieroids in general. — ^After being once 

 adjusted to give accurate pressures, as already described, the aneroid 

 should be handled with great care. Violent knocks and shaking 

 will, especially with the common aneroid, almost certainly change 

 or shift the various links and levers in their joints and change, more 

 or less permanently, the position of the index. For such reasons 

 aneroids are very liable to acquire unknown and often large acci- 

 dental errors, and can not, therefore, be regarded as very satisfactory 

 instruments. 



56. Errors due to very' slow chancfes^ " creeping.'''' — If an aneroid 

 adjusted to read correctly under ordinary air pressures is placed 

 within the receiver of an air pump, the index will quickly fall to a 

 lower pressure when a partial vacuum is formed. If, however, the 

 vacuum be maintained constantly at the same pressure for many 

 days in succession, the reading of the aneroid will be found grad- 

 ually to become lower and lower, but after three or four weeks 

 further changes cease or are very small. The amount of this slow 

 change differs greatly and may be from one-half inch or less to over 

 an inch, according to the diminution of pressure and other circum- 

 stances. Again, when the barometer is removed from the air pump 

 it does not immediately return to its original correct reading, but its 

 indications will be found to be too slow, several weeks being again 

 consumed in a slow return to approximately its former correct 

 reading. 



This " creeping " action depends, no doubt, upon some molecular 

 changes, as yet not clearly understood, that take place within the 

 materials of the aneroid box and steel springs. In any case the read- 

 ings are liable to be very seriously in erro, and tourists and others 

 who carry with them aneroids for the purpose of ascertaining eleva- 

 tions should have means to determine and eliminate the very serious 

 errors referred to above. A further discussion of these errors will 

 be found in the Monthly Weather Review for September, 1898, 

 page 410. 



57. The aneroid barometer is a convenient instrument for showing 

 more or less accurately the character and the amownt of harometric 

 changes going on from day to day, but the mercurial barometer is the 

 only instrument that gives atmospheric pressures with that degree 

 of precision required in simultaneous meteorological observations. 



58. Test of condition of aneroid. — Aneroids, seemingly good, are 

 often defective, because some of the joints of the levers and pivots 

 are too tight, causing the hand to stick and not move with the perfect 

 freedom it should. The condition of an eneroid can be quickly tested 

 in this respect by tapping the instrument on the side or bottom with 

 the fingers or knuckles, or perhaps better by lifting the instrument 

 about one-fourth of an inch from a table or cane-seated chair and 

 placing it back again somewhat sharply. Under this treatment, if 



