100 Ml!. W. N. SHAW ON HYGROMKTRIC METHODS. 



DEW-POINT HYGROMETERS. 

 1. RKGNAULT'S HYGROMETER. 



16. I pass on now to consider the observations rmicle with REGNAULT'S hygrometer 

 upon air in a known state of humidity. The method by which the thimble was 

 exposed to the air to be investigated has already been described ( 8). 



I need now only give particulars of the apparatus adopted for cooling the ether and 

 for reading. The thermometer had a very short cylindrical bulb, and was well 

 covered by the ether in the thimble. The stem of the thermometer was passed 

 through a cork fitting into the glass tube which held the thimble, and through the 

 cork passed also two very narrow copper tubes, one to the bottom of the ether, the 

 other just through the cork. The end of the latter was connected to a MAGNUS 

 aspirator, which was provided with a tap, so that the rate of passage of the air was 

 under very easy regulation. As the thimble was enclosed in a glass globe, it could be 

 viewed quite closely without any fear of the presence of the observer altering the 

 dew-point. 



17. So far as I can gather from published accounts, REGNAULT'S instrument has 

 always been regarded as a standard (see Note A, p. 130); that is to say, a dew- 

 point determination has been held to give a final verdict as to the pressure of vapour 

 in the atmosphere. REGNAULT'S directions for the use of the instrument are, first of 

 all, to cool the ether so as to obtain a deposit, and note the temperature ; then let the 

 temperature rise until the deposit is gone ; then cool slowly by tenths of a degree 

 until two-tenths are determined, for the lower of which there is a deposit and for the 

 higher no deposit ; the mean gives the dew-point, from which the pressure of vapour 

 in the air is at once obtained by reference to the table of vapour pressures. No calcu- 

 lation is necessary on account of the temperature of the air, because a change of 

 temperature of the air would not cause any appreciable change of pressure, either of 

 dry air or vapour, but only a change of volume. With regard to this method of 

 observation, I may remark that it is not always possible to confine "deposit" and 

 " no deposit" temperatures within the limits of a tenth of a degree. It is compara- 

 tively easy to do so when the circulation of air in the neighbourhood of the polished 

 surface is brisk, and I have found from some experiments on dew-point instruments 

 in the strong current produced by a rotary fan that the air current simply improved 

 the facility of reading the instrument, and did not seem to alter the final reading ; but 

 this opinion is, I believe, not shared by all experimenters, and I do not pi-ess it now, 

 as my observations assumed to some extent the permanence of the condition of the 

 air operated on.* In the experiments I am now dealing with the circulation was 

 slow. The thimble of the hygrometer was contained in a globe of 5 cm. radius, 

 holding therefore about half a litre ; through this 36 litres of air were passed on an 



* On this point, see below, pp. Ill and 142. 



