20 PROFESSOR CONNELL ON A 



the washers of the different screws do not become too dry. This is prevented by 

 the occasional use of olive-oil ; and this is one of the first things to be looked to, 

 at any time when the instrument may seem not to work well. It must of course 

 be remembered that the different parts of the instrument must not only be of the 

 best construction at first, but must be maintained in a fit condition, by constant 

 attention to the state of the valves, of the connecting screws, and of the piston, &c. 

 Every one must, of course, be allowed to make his observation as to the true point 

 of deposition, in the manner he thinks best ; but I may be permitted to make a few 

 remarks explanatory of my own views upon the subject. It is well known that it has 

 been often objected to Mr Daniell's instrument, that it gives the dew point too 

 high ; and Mr John Adie, by a comparison of its indications with those obtained by 

 D Alton's method, found the error occasionally to amount to (3 V\ and on an average of 

 28 observations, to reach 2 9. This is explained on the idea that the surface of the 

 ether, which is the seat of greatest cold, communicates the effect to the surround- 

 ing zone of the glass bulb of the hygrometer, before the bulb of the thermometer 

 has been cooled to the same extent by the liquid ; that portion of the bulb above 

 the surface of the liquid itself, as well as that below it in the liquid, being sup- 

 posed to be at a higher temperature. It appears to me that the instrument de- 

 scribed in this paper will be much less liable to such an objection, because metal 

 being a much better conductor of heat than glass, it is hardly possible that the 

 cooling effect should accumulate in any one zone of the little brass ball, but must 

 be diffused over the whole without delay. Time is thus given for the frigorific in- 

 fluence being communicated to the whole bulb of the thermometer, both by the 

 evaporating surface, and by the body of the liquid itself, which principally yields 

 the latent heat required by the ethereal vapour. I have already stated that the 

 thermometer ought to be so placed as to have the upper part of its bulb in the 

 plane of the surface of the liquid, or a little above that position, and the rest of it 

 immersed in the fluid. These observations being premised, I conceive that the 

 great point, in the first instance, is to endeavour to mark the very first decided 

 deposit of moisture on the outside of the little bottle. I am aware that some 

 regard the point of disappearance of the dew, when the cooling process is stopped, 

 as giving better indication. I confess I do not concur in this idea, because a 

 little time must elapse before the air can take up again what it has already de- 

 posited ; and if more than the very initial deposition has occurred, this taking up 

 will be still farther retarded, and the apparent point of deposition elevated 

 beyond the reality. This observation, of course, does not apply to Dalton's 

 method by transference, because each observation is isolated and complete in 

 itself. Most experimenters now, I believe, with ordinary dew point hygrometers, 

 take a mean of the two observations. This is perhaps the best mode of any, and 

 at all events is decidedly better than the disappearance alone ; and, accordingly, 

 it is the method which I have followed in making the observations with this 



