10 



ANEROID BAROMETER. 



reading off at the upjier statiou only. This cannot give true results, as the 

 scale can only he correct when the zero is in the position in which it was 

 graduated. Aneroids, with ^tixcd scales, are correct only at one given 

 temperature, and at one only. It is true that, by using the aneroid as 

 an ordinary barometer, and reading off in inches, we can, by a long calcu- 

 lation, arrive at a correct result, but when some hundreds or thousauds 

 of observations have to be made, the process of reduction will evidently 

 be a wearisome one ; and if we can read off correctly in feet, at ouce, 

 it will be a great advantage. 



The accompan}ang woodcut shows Field's aneroid as an instrument 

 2'Jin. in diameter, and a little more than an inch in thickness, thus fitting 

 easily in the waistcoat pociiet. Its *iovelty consists in the fact tliat the 

 ^ lid marked A A is moveable, and bears 



^— .-<^v-i7., ) nicks, W'hich h ck with a pin on the fixed 



y^^-,'^ . " *r«5^ lower case. These nicks are marked from 



■ '^^^'■^1-^-^'*' ' >V ^^° ^' ^° '^'^° ■^•' ^"'^ ^^^^ ^"^"^ must be moved 

 ?mS\/^ y^^^"^^ I'ound until the nick bearing the tempera- 

 ■ '<?> / (Jn^^"^ |.y^.g qJ ^]jg air at the time of observation 



is ti.xed by the lock-pin. The scale of feet 

 on the inside of the rim is then correct for 

 -aJk£>J^Maximmf^fQ ////that temperature. The altitudes are in all 

 ""^ cases determined by taldng two readings, 



,7^j<c^ /«■ one at the lower station and another at 

 c-^' .'^ the upi:)er, and then subtracting. 



The principle, in fact, is this: a n 

 Fig. 1. ordinary aneroid scale is con-ect at one 



temperature only, and is incorrect at ever}' other; but "the very 

 fact of the scale becoming inaccurate for the temperature for 

 which it was graduated, renders it practically accurate for some other 

 temperature, so that the shifting of the scale into certain fixed positions 

 answers the same purpose as if the original scale were altered to suit 

 various temperatures of the air." 



Another improvement which has been introduced has been the ' 

 jewelling of the working parts of the interior like a w'atch, the effect of 

 which has been to increase its sensitiveness in a marked degree, and to 

 render unnecessary the repeated tappings to which an ordinary aneroid 

 must be subjected. It has further been tested and verified at Kew 

 Observatory. A certificate from Kew should bo demanded by every 

 purchaser of meteorological iustruments, of whatever description. The 

 aneroid thus constructed is not intended to measure veiy great 

 elevations ; indeed, its sciile does not extend above 5, 000ft., but this 

 permits of reading with accui-acy, by means of a pocket magiiifier, to 

 2ft. or 3ft: To set it, we obtain the temperature of the air, by means 

 of a small whirling thermometer, ( thenniniiftrc froiidf,) which is slung 

 round the head by a cord, thus giving true shade temperature even 

 under the direct rays of the sun ; but this is really hardly necessary, 

 as if it be set approxinuitely within 5° F. a nearly correct result will 

 be obtained. 



The following instances nuiyhc taken as a fair test of the accuracy and 

 simplicity of this aneroid : — , ' 



August iHtli, ]H77. Temperature of air, 65° F., and scale set 

 accordingly. 



Feet. 



Jouruoy to Derby.- Reading at Leicester Museum 2,107 



,, Town Hall, Derby 2,0;)2 



Difforeiice 75 



