208 W, MARRIOTT — REDUCTION' OF METEOROLOGICAL OBSERVATIONS. 



when there are several of the same number together, except the 

 first and last. In no other case, however, should any figure be 

 omitted in any column, even though it be a cypher, as it may there- 

 by cause an error in the adding up. In taking the sums or totals of 

 the columns for the month, a convenient and simple method is the 

 use of constants, or adopting some number common. For instance, 

 in the barometer column it will be readily seen what is the 

 prevalent number, 29 or 30 ins. ; if 29 be taken as the constant, 

 it will only be necessary to add up the figures to the right of the 

 decimal point, and then to this total to add 1 for every inch above 

 29, and deduct! for every inch below. Example : If the total of 

 the 3 columns to the right of the decimal add up to 21 '648, and 

 to the left of the decimal 30 ins. occurs 7 times, and 28 ins. 

 3 times, the other values being 29 ins. ; then, by adding 7 for the 

 30 ins. and deducting 3 for the 28 ins., we get the following 

 result: 7— 3 = 4+21-648=:25-648 ins. This would, therefore, be 

 the total for the barometer column, if 29 ins. were taken as the 

 constant. Dividing the above figures by 31, which we will pre- 

 sume to be the number of days in the month, and adding 29 ins., 

 the mean will be 29*827 ins. In the same way, constants may 

 be employed in adding up the dry, wet, max., and min. thermo- 

 meter columns. This method should be adopted, as it is the means 

 of saving much time and labour, besides reducing the liability to 

 error. Great care should be taken that the adding up is correct, 

 and it is therefore desirable that the columns be added twice, first 

 upwards and then downwards ; so that if a mistake be made one 

 way, it will most probably be found out the other way. The 

 register should be complete, but if there be any omission in any of 

 the columns, notice must be taken of it in obtaining the mean ; 

 instead of dividing by the number of days in the mouth, it will 

 then be necessary to divide by the number of observations. In 

 dividing the sums to obtain the mean, the last figure should always 

 be increased if the remainder is more than half. 



The mean temperature of the month may be obtained by adding 

 together the dry-bulb readings at 9 a.m. and 9 p.m., the maximum, 

 and the minimum, and dividing the sum by four. 



