88 



MR. W. N. SHAW ON HYGROMETRIC METHODS. 



The ends of the connecting tubes coming through the mercury cups were moreover 

 covered with little glass caps between the observations to protect them from damp 



during that time. 



Fig. 5. 



One-tenth natural size. 



With these precautions it was assumed that any deposit received in the second 

 tube ought to be added to that in the first tube, and this departure from the practice 

 adopted in the first series of experiments is further justified by the fact that the new 

 aspirator, which will be described presently, more than doubled each of the old ones 

 in volume, so that the tubes had to take up twice the quantity of moisture (for the 

 same temperature) and in the same time as that required for the smaller amount in 

 the previous series. The variable amounts of moisture received by the second tube 

 showed that this part could not be neglected (see 15). 



iii. The aspirator. The pattern of this was similar to those already described, 

 but its volume was more than twice as great, and provision was made for increased 

 accuracy in reading ; it is accordingly figured in fig. 6. Between it and the rest of 

 the apparatus there was, moreover, a water-pressure gauge attached, which was found 

 very useful, first, as a test of the tightness of the joints of the apparatus ; and, 

 secondly, to see that communication was quite free between all the parts. The larger 

 aspirator was used, as it was intended to supply in some cases comparatively dry air, 

 and a larger volume was required to avoid the effect of errors of weighing. 



