4^8 Mr Foggo ati the Dew-Point Hygrometer. 



sides, granting that the incipient deposition has been observed, 

 what is the evidence that the exact temperature of this coin- 

 cides with that read on the thermometer ? The dew forms 

 principally in a ring parallel to the surface of the ether, be- 

 cause that is the line of greatest cold, its constituent tempera- 

 ture, therefore, is that of the superficial plane, while the bulb 

 is plunged below this into the body of the ether, which will 

 have a considerable influence in modifying its temperature. 

 The objections to the simple form thus recoil with increased 

 effect upon the original ; and if Mr Daniell is still incredu- 

 lous, he is doubtless prepared to explain the discriminating 

 faculty enjoyed by Newman's thermometers. 



I do not wish to prejudice any one by these observations 

 against Mr DanielPs hygrometer, the value of which is esta- 

 blished by extensive experience, and by high scientific autho- 

 rity. I only claim for the other the attention to which it is 

 entitled, and to show that in this case, as in every other, the 

 merits of a scientific question are to be decided by facts only, 

 not by cavilling on theoretical grounds, or by misapplied ridi- 

 cule. My conviction of the utility of this instrument is 

 founded on repeated trials of its performance made in com- 

 pany with its inventor ; and since the publication of Mr Da*- 

 nielPs opinion of the London and German instruments, I 

 have taken every opportunity of putting it to the test. 



As an infallible standard by which to judge of it, I com- 

 pared it with the results obtained by Dalton's method de- 

 scribed above ; and, at the same time, I used hygrometers of 

 various sizes and forms, but the agreement of them all was 

 complete ; and, excepting their superior delicacy, I could never 

 discover the most trifling discrepancy between them and the 

 standard of comparison. In the course of these examinations 

 the temperature has varied from 80° to 20°, the degree of 

 dryness ranging between saturation and a thermometric dif- 

 ference of 25° or 30®, and I frequently exposed the hygrome- 

 ters to the sun till they had risen 50 or 60 degrees above the 

 temperature of the air, but the application of the ether always 

 brought out the same dew-point. The construction I find 

 most convenient is a straight thermometer, carrying 15 de- 

 grees to one inch on its scale, the bulb projecting beyond the 



