162 LIEUT.-COLONEL SYKES ON THE ATMOSPHERIC TIDES 



without the aid of a second barometer : but they are very troublesome to fill ; are 

 destitute of a thermometer near the cistern, to determine the temperature of the 

 mercury; and want the means of adjusting the lower level of the barometric 

 column ; the tubes are frequently breaking, from the pressure of the iron screw 

 which fixes the cistern to the tube, (I have broken seven tubes from this cause,) 

 and in case of not being tightly screwed on, the cistern falls off from the weight of 

 the mercury in it, and the mercury is lost ; and from the uncertainty of expelling 

 air and moisture from the tubes, particularly in the moist months, the indications of 

 the instrument can only be looked upon as approximations to the truth. 



On the 12th of April 1827, I had the gratification to receive three barometers 

 from England : they were made by Gary on the Englefield construction, which 

 admits of a most delicate adjustment of the lower level of the barometric column 

 in the reservoir. They were beautifully finished, but unluckily had reservoirs of 

 ivory ; and I instantly foresaw the inconvenience to which such selection of ma- 

 terial would subject me. In the dry weather the ivory contracted, and permitted 

 the escape of the mercury by the screws (male and female) which joined the two 

 portions of the reservoir. Subsequently the reservoirs cracked at the spots where 

 the metallic screws attached the reservoir to the brass cylinder surrounding the tube 

 of the barometer. I was finally compelled from these disasters, within a twelve- 

 month, to send two barometers back to England to have glass or iron reservoirs put 

 to them. From the ease, accuracy, and delicacy with which the contrivance in these 

 instruments permits the mercury to be adjusted at its lower level, they require only 

 an iron cistern to render them quite eflficient ; and they are peculiarly suited to 

 measure minute changes in the atmospheric tides. Mr. Newman of Regent-street 

 has acted upon my suggestion, and has constructed two Englefield barometers with 

 iron cisterns, to which he has applied an excellent improvement of his own to pre- 

 vent the oscillation of the mercury in the tube en route. 



Having broken the seventh and last tube belonging to Jones's barometers, to pre- 

 vent my observations being confined to a solitary instrument I had recourse to one 

 of the India Company's barometers made by Gilbert: it was very heavy, and clum- 

 sily constructed, had air in the tube, and I ascertained the mercury not to be of 

 the specific gravity engraved on the reservoir. The instrument had a glass reservoir, 

 and the manner of fixing it to the tube was sufficiently ingenious ; but it wanted an 

 accurate and efficient method of adjusting the lower level of the mercury. This 

 operation was to be effected by looking through the glass reservoir and screwing up 

 the mercury to a line marked on it ; but the oxidation of the mercury usually 

 dimmed the glass, and made it no easy task ; even had it been readily practicable, 

 the occurrence of the tube exactly in the centre of the convex surface of the mer- 

 cury prevented its outline being fully seen, and the reading off could never be rigidly 

 accurate. These causes combined to render unsatisfactory, observations taken with 

 the instrument to fix the exact time of the flux and reflux of the diurnal and noc- 



