46 
On Hygrometnj. 
ateiv give us m-ic ~ . • . * .. . • . 
ing the specific gravity at 212°, and collecting for the temperature of deposition, 
the actual weight of moisture is easily deduced. 
Mr Dalton s method of determining this temperature, is the same as the expe- 
riment. of LeKoy, before alluded to. Writer contained in a common drinking glass 
was cooled down, either bv d ssolv ng saline mixtures in it, or ice till moisture is 
observed to be deposited on the sides of the glass The temperature of the water 
at this moment, as gi-en tiy a thermometer constantly immersed, is taken as that 
of saturation, and the tension as well as weight of aqueous vapour due to it, as that 
of the vapour actually existing ill the air. It is evident, that the solution of the 
problem may lie thus rendered quite perfect, guarding against such sources of 
fallacy as are sufficiently obvious : the only desideratum seems to lie some more 
ready method of arriving at the conclusion. The process described, though abund- 
antly simple, requires still a little time and attention. It also requires, that the 
observer should be provided, at all times, with some method of cooling the vessel or 
substance, on which the moisture was to lie deposited. This want of some more 
ready as well as portable apparatus, must have led the way to Daniel s hygrome- 
ter, an instrument founded on tile above principle, and which appears to hare 
completely answered the inventor s expectations, in the temperate climates of 
Europe. 
Even in Ettrope, however, it would seem to he attended with much the same 
inconvenience as the older and cheaper apparatus, imposing as it does, on the observ- 
er, the necessity of carrying with him a supply of ether, and requiring the stme 
time and attention in performing the experiment. The first objection is not 
perhaps important in the more civilized countries of Europe, but in India it is 
otherwise. Here we must always prefer the apparatus and ingredients, which are 
most easily replaced or replenished; and consequently, saltpetre and sal ammoniac, 
which may he had in every bazar, are preferable to the ether required for Mr. 
Daniel’s instrument. On the score of economy too, there can be no comparison be- 
tween the two methods. But there is a still more serious objection to the use of Mr. 
Daniel s hygrometer in India. It is the almost total want of action under which it 
labours, o a ing to the impossibility, I suppose, of keeping our ether in anv thing like 
a state of purity, in temperatures which for some part of the vear pWrw’.l tl.nt nf its 
