244 Horary Observations of the Barometer, fyc. [April, 



Note. — On referring to my manuscript meteorological table for the 

 month of March, I find that an error of -05 has been made in the 

 printed entry of the two barometers at 10 a. m. on the 21st, which 

 should stand 29,899 and 29,947. After correcting these, it will be 

 found that to reduce Mr. Barrow's observations to terms of the baro- 

 meter I have hitherto registered '015 must be added to the corrected 

 column at 32° : and to compare them to the new standard by New- 

 man *029 must be deducted. 



These discrepancies are nothing more than index errors ; but as it 

 is a matter of some importance to know which gives the correct alti- 

 tude, and why an instrument commissioned with such precautions from 

 the best maker at home should stand three or four hundredths of an 

 inch lower than tubes made, filled, boiled, and measured in Calcutta ; I 

 have with Mr. Barrow's aid remeasured the scales of the several 

 instruments respectively from to the 30 inch mark, by a standard 

 brass scale of Troughton's at the temperature of 95°. 



Mr. Barrow's scale was laid off by himself exactly 30,000 inches. 



My compensation barometer to a scratch on the glass origi- t 



nally marked by myself with the same care, was found on L 30,000 



remeasurement to be quite correct J 



Newman's Strd. 1st trial 29,658 + 1,176— 0,814 = 30,020 -, 



2nd do. 28,746 + 1,176 + 0,100=30,022 1 30,020 

 3rd do. 28,848 + 1,176 =30,018 J 



The principal difficulty in measuring the column of Newman's in- 

 strument was to find the distance from the lower end of the ivory 

 cone (or the level of the mercury in the cistern) to the upper part of 

 the cistern : this I made by several trials 1,173 to 1,176 ; Mr. Barrow 

 made it 1,182 and 1,183; Mr. Pearson 1,172: I have taken it at 

 1,176 as the mean, and feel confident the error of the whole measure- 

 ment does not amount to 0,005 inch. The readings therefore of this 

 instrument in every instance will be ,020 too low. 



I am reluctant to suppose Mr. Newman should have sent me a 

 barometer at such a vast cost so carelessly verified ; but such seems to 

 be the case from the above measurement, which is confirmed by the 

 register ; for allowing ,009 for the expansion of the brass scale, and 

 adding it to the index error above, we find almost the exact amount 

 by which the new instrument stands lower than my former standard 

 which latter has been compared by three opportunities with the Royal 

 Society's barometer and found to agree very closely. Mr. Newman 

 neglected to make this comparison, although I particularly requested it. 



J. P. 



