488 Proceedings of the Rot/al Irish Academy. 
the latitude of Dublin by the Rev. Dr. Dixon. Divide that elasticity 
into the height of the barometer. Divide the quotient into the weight 
of a cubic foot of aqueous vapour, at the temperature of the dew-point: 
the quotient will be the grains' weight of water contained in a cubic foot 
of the atmosphere, at that time. 
Thus, at the dew-point 60°, the elasticity of vapour is 0-5178, and 
this elasticity divided into the barometric height 30 gives 58, and 
dividing this 58 into 372, the weight in grains of a cubic foot of satu- 
rated vapour at 60°, gives 6*4 grains as the weight of water contained 
in a cubic foot of the atmosphere or space, at that time, under the cir- 
cumstances stated. 
This computation shows what weight of aqueous vapour would be 
found in a cubic foot of air or space if the vapour were saturated, but 
it is almost never saturated, as is well known to those who have long 
observed the psychrometer, the two bulbs of which almost never agree. 
In De Luc's experience, he found them to agree but once. During my 
own observation of an excellent psychrometer, I never found the two 
bulbs to agree exactly, the difference varjing from half a degree to 10 
or 12 thermometric degrees, after six or eight hours of incessant rain; 
and other observers have found the difference much greater. This 
hygrometer shows how far the air is from saturation, that point being 
the completion of the tenth round of the index: the fraction of satura- 
tion shows how much has been already accomplished of the computed 
quantity of water, and by the distance of the index from the completion 
of the tenth round, how much is yet to be accomplished. 
The computation shows how much saturated vapour of water the 
cubic foot would contain ; the hygrometer shows how much it does con- 
tain, and also the deficiency — i. e. the quantity which, if added to that 
actually contained, would make up the computed quantity. If the 
index went ten times round the dial, the atmosphere would be saturated 
and could hold no more, that being the extreme of moisture. But as 
the air is, we may say, never saturated, the index will not turn round 
ten times, unless placed in an artificial atmosphere of aqueous vapour. 
On examining the state of the hygrometer at any time, we find perhaps 
that two spherules have been thrown off, and that the index points to 
several degrees more. Suppose both together indicate 250°, we learn 
that the atmosphere is one quarter saturated, and as computation has 
shown that 6*4 grains of water would be saturation at 60°, and as 
one quarter only of the saturating quantity is present, we learn that 1 J 
grains of water are suspended in vapour at that time, in every cubio 
foot of air or space. 
In fine, whatever maj^ be the fraction of saturation presented by 
the hygrometer, that will be the fraction necessary to make up the 
weight of water which computation has shown to belong to that tem- 
perature when saturated. 
A few suggestions relative to the care of the instrument may be 
useful. The compound line should not be kept tight when not in use; 
