1836.] 



Horary Observations taken at Dddupur. 



301 



Memoranda relative to the above Table of Barometrical observations for Septem- 

 ber 1835. By Lieut. Durand. 



Instruments employed. — Colonel Colvin's Barometer was made by Gary. 

 The cistern is of ivory, the Instrument is adjusted for observation by 

 bringing the surface of the mercury to a level with the slit in the socket 

 of the stopcock of the cistern, by means of a brass screw at the bottom of 

 the cistern. This instrument was, when compared with the standard 

 Barometer in Calcutta, by J. Prinsep, Esq., found to be correct. 



Lieut. Durand's Barometer is one of Troughton and Simm's mountain 

 Barometers. When compared with the standard Barometer, it stood 0.043 

 too low. 



In order to determine the amount of change which the two instruments 

 might have suffered, relatively to each other, in consequence of the jour- 

 ney from Calcutta, a comparison was instituted between the heights shown 

 by the two Barometers, during the month of June 1835 ; the following is 

 the result. 



Mean. 

 Height of Bar. 



28.744 

 28.6488 

 28.7486 

 28.6585 



Attd. 

 Ther. 



90.4 

 95.44 

 87.83 

 89.4 



Detd. 

 Ther. 



89.51 

 94.25 

 86.51 

 87.96 



Time. 



10a. » 

 4 p. M 

 10 a. m 



4 P. M 



Mean of the foregoing. 



'28-69441 92.92 j 91.83 184.145 Cary's Barometer. 



28.703sl88.616 87-236) 83. Troughton and 

 Simm's Barometer. 



Whence may be deduced, that Gary's barometer suffered considerable 

 derangement from the inevitable jolting, &c. attendant on so long a jour- 

 ney. Troughton's had evidently been less disordered, probably but little 

 so ; without a second comparison with the standard barometer, however, 

 the comparative accuracy of Troughton's rests upon supposition. 



Previous to the day on which the hourly observations were to commence, 

 Cary's Barometer was accidentally put out of order, and it became requi- 

 site to re-fill the tube with mercury ; this was accordingly effected, but 

 the means for safely heating the filled tube not being at hand, and the 

 tube appearing to the eye free from air, it Mas inserted into the instru- 

 ment ; the observations show the great difference which this untimely 

 accident caused in the heights of the two mercurial columns. 



Thermometers. — The attached thermometers of both barometers read 

 off to degrees. 



The thermometer employed as a moist bulb, is one made by Trough- 

 ton and Simms. When compared in Calcutta it stood 1*3 too high ; the 

 scale is graduated to degrees. 



The detached thermometer also by Troughton and Simms, reads off 

 only to two degree divisions; when examined in Calcutta it was found to be 

 0*4 too high. 



From the foregoing remarks on the thermometers, it is evident that the 

 division of the scales of these instruments did not admit any perfect accu- 

 racy in reading off the decimal parts of a degree : the decimal parts in the 

 table are therefore only careful approximations, and under particular cir- 

 cumstances, such as reading off at night, &c. small inaccuracies must have 

 been unavoidable. 



Time. — The time of apparent noon and the rate of chronometer were 

 ascertained by a series of observations of equal altitudes of the sun, on the 

 1/th, 18th, 19th and 20th September. 



Location of Instruments. — The instruments were placed in a verandah 

 facing the north, perfectly shaded, and sheltered from the wind, without, at 

 the same time, hindering a free circulation of air. Cary's Barometer stood 

 about 4ft. 6, from the wall; Troughton's about lft. 6, from the wall, the 

 space partitioned off and allotted to the instruments not admitting their 

 further removal from the northern front of the building. 



Place of Observation. — Dddupur is situated on the right bank of the 

 Jamna, a little below the junction of the Sombe : the positiun of the Canal 



