26 Dr. Masons Deacrijjtion 



a volume on the Medical Topography, climate, physical 

 structure, and past and present condition of that island, 

 now preparing for the press by my friend, Mr. Blewitt, 

 and myself; being convinced that medical practitioners do 

 not yet possess that precise knowledge of the climate of 

 Madeira which is necessary to enable them to decide on the 

 particular classes of disease to which it is adapted : — I was 

 also convinced that an instrument which would at any time 

 indicate with facility and precision the actual state of the 

 air in regard to humidity or dryness, would be a valuable 

 acquisition to science. My first idea was that the instru- 

 ment would only serve to indicate the relative humidity 

 of the atmosphere, or its distance from the point of satura- 

 tion ; but after considering all the laws of evaporation, and 

 taking a general view of my own experiments on the sub- 

 ject, I have been able to find the dew-point by very simple 

 calculation, and also to trace out the relation between this 

 instrument and Sir J. Leslie's. Considering that we ought 

 ever to bear in mind, that nature is constant in her opera- 

 tions, and that the same causes invariably produce the same 

 effects, it hence became easy not only to obtain all the indi- 

 cations that I required from the instrument itself, but also 

 to ascertain by it the results given both by the dew-point 

 Hygrometers and by that of Sir J. Leslie, with perfect 

 accuracy and invariable certainty ; so that it may be truly 

 regarded as supplying the place, if not of superseding alto- 

 gether, the more complex instruments which have pre- 

 ceded it. I shall subsequently enter more fully into the 

 consideration of the important fact, that any errors which 

 may arise in taking the temperature of the shade, will in 

 my Hygrometer give an excess of moisture, while those of 

 the dew-point Hygrometers will indicate an equal excess of 

 dryness ; so that when both instruments are used, if very 

 accurate data be required in ascertaining the absolute 

 weight of vapour in a given quantity of atmospheric air, 

 the mean of both instruments will insure the truth. 



I will here state all the objections which have been ad- 

 vanced against the principle of Hygrometers by evaporation 

 which Dr. Hutton first proposed, the chief of which have 

 been grounded upon the belief that the air dissolved the 

 vapour contained in it in a chemical manner, the same as 



