METEOROLOGICAL OliSERVATIONS. 201 



"what is the average correction, — in the present case it is '26 ; tliis 

 might be called -3 ; and by deducting -3 from all the values in 

 the left-hand column, and entering the results in the right-hand 

 column, the reading at the station corresponding to that at the sea- 

 level will be approximately obtained. This is the column to be 

 used for ascertaining the correction to be applied to the barometer 

 reading to reduce it to sea-level. 



The following example will show the method of applying the 

 foregoing corrections. Suppose the readings to be : 



Attached thermometer. Barometer. Dry-bulb. Correction for index error. 

 55° 29-526 ins. 53° —-005 in. 



then ins. 



Barometer reading ... ... ... ... ... 29-526 



(1) Correction for index error ... ... ... — -005 



(2) „ temperature 55° (Table I.) 



29 



29 

 (3) „ altitude (Table III.) + 



Barometer reading at sea-level ... ... ... 29 



521 



070 



451 



259 



710 



As the application of these several corrections takes some time, 

 I have devised a form of table for combining all three (when that 

 for index error is the same throughout the scale), which is the 

 means of saving much time and labour, besides greatly reducing 

 the liability to mistake. It will be seen in Table III. that a 

 change of 0-6 in. in the pressure produces the same amount of 

 variation in the correction for altitude as is produced by an altera- 

 tion of 10° in the temperature of the air. For instance, the 

 correction for altitude, when the pressure is at 30-0 ins., at the 

 temperature 60°, is the same as that when the pressure is 29-4 ins. 

 and the temperature 50°. By taking a mean height of the barometer, 

 we can combine the correction for temperature with that for alti- 

 tude ; and when the index error is constant (as in the Fortin 

 barometer), it can also be included. Table IV. is a specimen of 

 this form, made out for each -06 in. of barometer reading from 

 28-68 ins. to 30-18 ins. and every 2° of temperature of the drv-bulb 

 and attached thermometers, for a station 240 feet above sea-level. 

 To use the table : Look in the column on the right or left in the 

 upper portion of the table, for the reading of the barometer at the 

 station, and carry the eye horizontally to the temperature of the 

 air [i.e. dry-bulb reading), and then vertically downwards, and 

 through the corresponding column in the second half of the table, 

 until the value horizontally opposite the temperature of the at- 

 tached thermometer is the required correction. 



Thermometers. — The corrections for the various thermometers 

 (dry, wet, max., min., etc.) are given on the Kew certificate for 

 every 10°, and will most likely vary throughout their scales. They 



15* 



