76 MB. W. N. SHAW ON HYGROMETRIC METHODS. 



at the instant of the formation of the dew deposit. These differences may, perhaps, 

 best be illustrated by considering what would be their effects upon the ideal curve 

 which would represent the continuous variation of the hygrometric state of the air, if 

 we suppose the air to be subject to a series of rapid alternations of moisture and 

 dryness ; the dew-point instrument would give a series of points on the true curve ; 

 the observations with the wet and dry bulb would give a series of points, not on the 

 true curve, but a somewhat modified curve, the slopes being more gradual ; and, pro- 

 vided the alternations were sufficiently rapid, the curve of the wet bulb would 

 generally be smoother than the true curve. A series of observations with the chemical 

 method would give a series of points on a third curve quite different from the other 

 two, and one from which all effect of rapid alternations would have disappeared, and 

 only the more permanent changes would be shown. Hence, two instruments of 

 different kinds can only be compared by taking the mean of a number of consecutive 

 observations, and assuming that the effect of all rapid alternations disappears ; and 

 this is probably the case under all ordinary circumstances. This must, however, be 

 borne in mind in considering the comparisons, because the rapid alternation is 

 possibly of not infrequent occurrence. When using a dew-point instrument it is 

 sometimes observed that at the same temperature the dew forms and then disappears 

 again. This phenomenon has been differently interpreted by different observers, 

 some considering that it shows the extreme sensitiveness of the instrument to small 

 atmospheric changes, while others have attributed it to the variation of the observed 

 dew-point with variation of wind velocity (see p. 142). 



It is, therefore, evident that it would be of some advantage to eliminate if possible 

 these two causes of uncertainty. I have endeavoured to do so in the experiments I 

 am now communicating, in the following manner : The instrument to be tested was 

 enclosed in a glass vessel, B, which was connected on the one side with an apparatus, 

 which I will call a saturator, A, designed to saturate air at a given known tempera- 

 ture, and on the other side with drying tubes, C. An aspirator drew air through the 

 whole arrangement. The hygrometric state of the air is given (1) by the temperature 

 of the saturator, A ; (2) by the instrument ; (3) by the drying tubes, C. I first ascer- 

 tained ( 1 to 10) that when B was cut out and A and C put in immediate connexion, 

 the two methods namely, the saturation method and the chemical method gave 

 concordant results. Then B was introduced, but the dew-point instrument was not 

 worked, and the results of A and C were again compared and found to be equally con- 

 cordant. This showed that the state of the air was not altered by the mere presence 

 of B ; and, finally, observations were taken with the dew-point instrument while the 

 air was being drawn through, and the results of A and C were again compared. It 

 was found that they were still concordant, and since the state of the air was known 

 before it passed over the instrument, and was proved to be the same after passing the 

 instrument, we are quite safe in assuming the hygrometric state of the air while it 

 was passing the instrument to be that given either by the saturator A or by the 



