436 H. A. Hazen — Condensing Hygrometer 



received the same addition of heat as the outer surface of the 

 plate." There seems to be a slight misconception by the writer 

 of the above as to the amount of the effect under these con- 

 ditions. It is an undoubted fact that in the hands of an un- 

 skilled observer the results will be exceedingly faulty, but if 

 due attention be paid to the elimination of errors, a fairly con- 

 stant determination of the dew-point may be made again and 

 again and under widely differing conditions. 



The psychrometer has received even greater condemnation 

 than the instrument just alluded to. The following is quoted 

 from Symons's Magazine for May, 1885. p. 56, and is an extract 

 from a letter by Mr. Dines: " We have for many 3 r ears past relied 

 almost exclusively on the indications of dry and wet-bulb ther- 

 mometers to give relative humidity ; it is now admitted that ' all 

 deductions from these are open to doubt.' I should go further 

 than this and say that they ought not any longer to be used for 

 that purpose. It is only too true to say that, out of the mass 

 of observations which has been accumulating for the last few 

 years, we have no data upon which we can rely to ascertain to 

 what degree of humidity the air attains in this country, and 

 which is of far more importance, to compare the dryness of the 

 air at one place with that at another; even at times when both 

 thermometers read alike it does not follow that the air is satu 

 rated with vapor, or in other words that its relative humidity 

 is 100 per cent. Objections have been made to the form 

 in which the figures composing a table of relative humidity are 

 given, but this is as nothing when compared with the manner 

 in which those figures are obtained. I should be sorry to say 

 one word against the use of the dry and wet thermometers, but 

 instances by the thousand show plainly that they are not fitted 

 to give the dew-point temperature, and it is to the persistent 

 use of them for this purpose, that Mason's Hygrometer, one of 

 the most simple and useful of all meteorological instruments, has 

 been brought into discredit." Here again there is great mis- 

 understanding; it is granted that, as ordinarily observed, the 

 psychrometer, in a very close shelter or on the north side of a 

 building where there is no ventilation, will give exceedingly 

 unsatisfactory results, but that is not the fault of the instrument. 

 It is hoped to show that with proper care the indications of the 

 psychrometer may be made very accurate, and, what is more 

 important, may be made nearly invariable, i. e., so that constant 

 results may be had under the same conditions. Unfavorable 

 criticisms like the above might be quoted sufficiently numerous 

 to fill a small volume. 



It would seem that, if a nearly uniform law can be estab- 

 lished between the indications of these two instruments, under 

 all conditions of temperature, dryness, motion and other con- 



