22 MADEIRA METEOROLOGIC. PART in. 



Watery Vapour. 



The simplest and quickest observational indication 

 of the degree of presence of this invisible water-gas 

 element in the air about us is, without doubt, the inverse 

 depression, as with Dr. Mason, of the wet below the dry 

 bulb thermometer, either at the instant or for the mean 

 of the day, or the month, or the yean Now, these last 

 means having been already given, we need only quote 

 in this place that the mean wet-bulb depression for the 

 year at Madeira is only 4/6 below what would be full 

 saturation ; while at Lisbon that point is removed further, 

 or to 6'i; and at Jerusalem to 9*5. In Scotland, 

 indeed, the wet-bulb depression is only 2*1, but at so 

 very low a temperature as to represent but a trifling 

 amount of absolute watery vapour really present. 



In fact, when this rather intricate matter is duly 

 computed out, it appears that Scotland has the least, and 

 Madeira by far the most, grains weight of invisible watery 

 vapour always present in every cubic foot of its atmo- 

 sphere of all our four chosen comparing stations. Yet 

 the air of Scotland is by no means, therefore, to be con- 

 sidered a dry air ; for, at its already alluded to low 

 temperature, the addition of only half a unit grain of 

 moisture would convert invisible water-gas into visible 

 mist, cloud, or fog ; while in Madeira three times as much 

 would have to be added before the same sensible result 



