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 DanteLw’s instrument, that it gives the dew point too 
high; and Mr Joun Avis, by a comparison of its indications with those obtained by 
Dauton’smethod, found the error occasionally to amount to 64°, 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. Iam 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 Dauron’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 
