471 



X. NOTES ON THE READINGS OF THE ASPIRATION PSYCHROMETER, 

 AND OF THE DRY AND WET BULB THERMOMETERS, AND ON 

 THE OBSERVATIONS OF EVAPORATION AND PRECIPITATION, 

 AND OF THE EVAPORATION OF ICE. 



BY 



W. H. DINES, F.R.S. 



A CONSIDERABLE number of observations relating to humidity, precipitation and evaporation were made 

 during the two years that the " Discovery " remained in the Antarctic regions. 



These consist of two-hourly readings of the dry and wet bulb thermometers, of a large number of 

 independent observations of the humidity taken with an ASSMANN'S aspirator, of measurements of snowfall 

 (there was no rain) in a rain-gauge, of measurements of the depth of snow by means of stakes driven in to 

 show the change of level, of evaporation from the surface of a frozen pond, from lumps of ice, and from 

 ice exposed in shallow metal trays. 



The value of the observations would have been greatly increased if more particulars had been given, and 

 one cannot help feeling regret that the opportunity to revise the humidity tables at low temperatures 

 could not have been utilised. On the other hand, the zeal shown, and the trouble taken by the officers of 

 the ship in making the observations, deserves the highest praise. 



The humidity observations consist of the readings of the dry and wet bulbs taken at the regular two- 

 hourly intervals. Gaps in the series occur, most noticeably during the first winter, the observations being 

 absent for the period May to September, 1902. During the second winter, many observations were made 

 with an Assmann aspirator. Great care seems to have been bestowed on these, and they form a very 

 useful chepk on the dry and wet bulb readings. The difficulty of obtaining accurate values of the humidity 

 when the temperature is below the freezing-point is well known even in our mild climate, and it must be 

 greatly increased when the temperature approaches the freezing-point of mercury. The general opinion 

 of meteorologists seems to be that the tables by which the relative humidity is obtained from the wet and 

 dry bulb readings are open to doubt at very low temperatures and very low values of the humidity, and 

 hence it hardly seems worth while calculating the values of the humidity from these readings. It is, 

 however, plain, from a study of the figures, that values of the humidity of less than 50 per cent, occur 

 frequently, and, quite apart from any doubt of the tables, there are many instances of extremely low 

 values. Hence one may say that whatever else the climate may be, it is certainly dry. 



The observations on precipitation were made with a rain-gauge, but there are no details given as to its 

 size and position, or the height of its rim above the surface of the snow. Precise observations are utterly 

 impossible in such a climate, indeed the observers state that during the blizzards they did not and could 

 not know whether the snow was actually falling from the clouds, or whether it was merely drift from the 

 surface of the snow. Often, too, the gauge had to be dug out of the drift. On calm days the amount of 

 snow was measured, but what proportion the fall on such days bears to the total fall it is impossible to say. 

 The whole precipitation took the form of snow, there being no record of rain, during the two years that 

 the " Discovery " remained at her Winter Quarters. 



An attempt was made, also, to measure, by means of stakes, the depth of snow that fell, the graduations 

 on the stakes showing the level of the upper surface of the snow. 



