t 



178 On Taking the Dew-point. 



greater number of cases of the dew-point which will fall under 

 our observation. By employing 1^ measure of the salts with 

 the same proportion of water as before, a few degrees greater 

 fall will be commanded; and with two measures of salt and one 

 of water, a still greater cold is procured if wanted. 



The point that chiefly requires attention is a moist state of the 

 atmosphere, of which the operator can easily judge by the ac- 

 tual or recent fall of rain, and by the general appearances of the 

 heavens. In such cases, it is necessary to use water of a tempera- 

 ture from 5° to 10° above that of the air, which is readily ac- 

 complished, simply by grasping the measure containing the wa- 

 ter in the hand for a minute or two before pouring it into the 

 brass bottle. In this way, the initial fall of the temperature is 

 so much retarded, that we can easily, during the subsequent gentle 

 agitation, mark the dew-point, even when differing little from 

 the temperature of the air ; and whenever we find that a too ra- 

 pid fall takes place at first, we have always this method in our 

 power as a corrective. 



On the ether hand, when very fine weather, a high barome- 

 ter, and other general appearance*, indicate a very dry state of 

 the atmosphere, it may be proper to employ \ \ measure of salts- 

 and one measure of water, to ensure a greater reduction of tem- 

 perature. 



The usual mode of operating will, however, be with one 

 measure of the salts and one measure of water, either of the 

 temperature of the air, or some degrees above it, as appearances 

 may suggest. After a little experience has given the requisite 

 dexterity in the manipulation, and a moderate degree of fore- 

 sight respecting the general state of the atmosphere, we shall 

 not often find it necessary to repeat the operation. In a series 

 of observations at the distance of some hours from one another,, 

 the preceding observation will of course be some guide to the 

 subsequent *. 



With the bally thermometer, and measures, and a small bottle of 



• In a very few instances, when the dew-point is high, the layer of salt 

 causes a deposition on the bottom of the bottle, before it extends to the sides. 

 This appearance, which occurs only very rarely, is to be overlooked, because 

 the thermometer is scarce affected by the cold which occasions it ; and we 

 must wait until, during the gentle agitation, we observe the deposition on the 

 middle part of the sides of the bottle, as we are then sure that the cold has- 

 been equally diffused, and the thermometer duly affected. 



