1823.] for Barometrical Measurements, S6t 



it be any other than 30 inches, is distinguished by a mark on 

 the scale.^ 



V. In forming the preceding compendium, one considerable 

 source of abridgment was found in omitting altogether many 

 details on the subject of the hygrometer. This I have been 

 induced to do from two reasons ; first, the methods described 

 by M. Ramond apply solely to the use of hygrometers on the old 

 constructions, and are both long, and probably inaccurate, when 

 compared with the more improved methods now generally 

 adopted on the principle of evaporation. Secondly, M. Ramond 

 himself only treats of them as connected with the stationary 

 meteorological observations. He conceives them of little use in 

 the measurement of heights as the following quotations will 

 clearly show : — 



(Second Memoir, § 3, p. 57.) — Allowing all that can reasona- 

 bly be done to the error of the instrument, it is still certain that 

 I have made observations at extremely different degrees of 

 humidity ; and that nevertheless the effect of this circumstance 

 has been covered in extraordinary cases, by that of more prepon- 

 derating circumstances by which they were accompanied ; and 

 in ordinary cases by even the allowance due to the error of 

 observation. The reason is evident : the factor for the temper- 

 ature having been empirically determined contains the correction 

 for the mean humidity ; and the quantities by which this mean 

 humidity has to be augmented or diminished, are ordinarily too 

 small to affect sensibly results, on which the least accidents pro- 

 duce a greater effect than these quantities. 



(Third Memoir, Part III. p. 99.)— It is well known that the 

 mixture of vapour diminishes the weight of air ; but we know 

 also the limits within which this action is confined; and if we 

 admit it into the number of causes which determine the varia- 

 tions of the barometer, we are not ignorant that it is far from 

 completely accounting for them. Even when atmospheric air is 

 susceptible of passing naturally to the state of dryness to which 

 we can bring it artificially, the return from this state to that of 

 saturation will only diminish the elevation of the column of 

 mercury, by from a sixtieth to a fiftieth part, according to the 

 temperature of the mixture ; but experience proves that the air 

 never approaches to absolute dryness, and that it always retains 

 a considerable dose of moisture, so that the usual variations in 

 this respect will scarcely account for a variation of a 120th or a 

 100th part. Now the oscillations of the barometer in our cli- 

 mate run through a space equal to at least 1-1 8th of the total 

 height ; and the barometer rises and falls frequently in the oppo- 





• Should the preceding remarks, or any others in this appendix, appear of a more 

 elementary nature than are usually the topics of discussion in scientific journals, the 

 author begs to state, that the whole was originally drawn up with a view to separate 

 publication ; and he conceived he was consulting the convenience of many readers in 

 giving the detail and reasons of every part of the operations. 



