36 EIGHTH REPORT — 1838. 



quently in the case of the transport of clocks with this description of 

 penduhim to great distances, either this part of the lyachine {which re- 

 gulates the tohole) must be placed in inexperienced hands, to be filled 

 with mercury and to have the column adjusted to the exact length re- 

 quired for compensation, or a workm.an must accompany the clock and 

 set it in action. The expense of the latter process constitutes, gene- 

 rally, a very serious objection. 



The pendulum cistern being made of glass, is liable to fracture, and, 

 on account of the risk which attends the boiling the mercury within 

 it, to drive off the air, this process is never attempted. It is very pos- 

 sible to give to a glass vessel, externally, a form matliematically coi'rect, 

 but the case is very different when similar accuracy is required for the 

 interior. The glass cistern, therefore, never receives a perfect figure, 

 and the mercurial column it contains cannot be a regular cylinder. This 

 condition, combined with the irregularity of expansion which glass is pe- 

 culiarly liable to from its compound nature, renders measurement and 

 calculation with regard to the column, so vague and deceptive that they 

 are ne^'er employed. 



In order to meet these serious obstacles to the satisfactory and ex- 

 tensive use of this valuable instrument Mr. Dent has recommended the 

 substitution oi cast-ii'on iov glass in the cistern. 



Mercury and cast-iron are quite as little disposed to amalgamate as 

 mercury and glass ; and iron is a material, compared with glass, which 

 is more simple in its nature, and more obedient to the workman. It is 

 susceptible of the most perfect forms, which it will maintain with very 

 little liability to alteration, and is quite proof against numerous acci- 

 dents that would be fatal to glass. The expansion of ii'on by heat being 

 also uniform and well known, it is evident that, in tlie cast-iron cistern, 

 we may have a vessel of a known, regular, and permanent figure, or, if 

 not strictly permanent, one whose changes and their laws we are acquaint- 

 ed with. Calculation may, therefore, be used in anticipating results, with- 

 out any fear of its widely differing from experiment. 



Further, — in a cistern of cast-iron, the mercury may be boiled at any 

 time. The clockmaker may do it himself when he first puts the machine 

 together, — he may adjust the column, — he may then hermetically seal 

 it, and despatch the pendulum to the most distant countries with the ad- 

 justment so perfect that it maybe instantly attached to the wheel-work 

 by any workman capable of setting the clock upon its supports. If, at 

 a subsequent period, minute portions of air have, from any cause, again 

 mingled with the mercury, and rendered the pendulum susceptible of 

 barometric changes, the air may be again driven off with the greatest 

 facility, by repeating the process of boiling, without removing the mer- 

 cury from the cistern. 



Mr. Dent has accompanied the introduction of cast-iron in the cistern 

 with several other alterations which have all the same intention of im- 

 proving this kind of pendulum. Among others, he has removed en- 

 tirely the metal stirrup, or frame, Avhich carried the cistern, and has at- 

 tached the latter to the rod ; — he has prolonged the rod, and plunged 

 jt into the mercury, nearly to the bottom of the cistern ;— a condition evi- 



