540 MR A. J. HERBERTSON ON 



differences between the readings of the two dry bulb thermometers cannot be explained 

 by different wind speeds, although such a factor must be taken into account in the case 

 of moistened bulbs. These differences really indicate different sensitivenesses of two 

 instruments exposed to similar variations in the atmosphere under slightly different 

 conditions. In persistent fog and steady strong winds the serial conditions remain 

 uniform and the temperature differences of the two instruments are small. This is 

 shown in the minute to minute records of table. 



On the other hand, in clear calm weather the physical conditions of the atmosphere 

 vary greatly and rapidly, especially at such a station as Ben Nevis, where air may come 

 from below as well as from all sides and from above.* No doubt under such 

 circumstances the two thermometers are not in identical atmospheres, but the minute 

 to minute observations indicate that on the whole their progressive readings have a 

 similar trend, but that the aspiration-psychrometer is more sensitive to change than 

 that shut in the Stevenson screen. 



This extra sensitiveness of the aspiration-thermometer is shown in the hourly 

 observations tabulated above, and some of its successive records may be examined 

 before proceeding to discuss minute to minute variations. 



A very remarkable instance of the value of the aspiration-psychrometer is seen at 

 19 hours on 2nd September 1893 at the summit station. A slight fog enveloped or lay 

 just above the summit at 1 6, 1 7, and 1 8 hours, but cleared before the 1 9 hours' observation. 

 The aspiration-psychrometer dry bulb rose from 6° "18 C. to 8° '52 C, and the wet bulb fell 

 from 6° '08 C. to 3°*56 C. The instruments in the Stevenson screen, however, were not 

 affected to this extent. The Stevenson wet bulb reacted to the drier air, falling from 

 6°'28 C. to 3°'89 C, but the dry bulb scarcely moved, rising from 6° - 28 C. only to 

 6°'67 C. 



At 20 hours the readings were : — 



Aspiration-psychrometer, dry, 7°'28 C. ; wet, 5°'41 C. ; Stevenson screen, dry, 6 0, 11 C. ; 

 ^et, 5° '11 C. 



This lowness of the Stevenson screen dry bulb is probably to be explained by the 

 dampness of the Stevenson screen. 



On 3rd September 1893, after a series of variations which can be correlated with 

 changes in the fog, the difference between the Stevenson and aspiration-thermometers 

 suddenly altered from + 0*65 at 16 hours to — 0'49 at 17 hours, owing to the fog sinking 

 after the 1 6 hours' observation. The warmer air caused the Stevenson screen temperature 

 to rise from 9°*00 C. to 12°'44 C, i.e. 3°'44 C, but it increased that of the aspiration- 

 psychrometer from 8°'35 C. to 12°'93 C, i.e. 4°'58 C. 



The minute to minute readings made during the present investigations afford an even 



more valuable means of comparing the relative sensitivenesses of the dry and wet bulb 



thermometers in the aspiration-psychrometer and the Stevenson screen during rapidly 



changing conditions of temperature and humidity. Taking Ben Nevis, No. 161, as an 



* Cf. J. Y. Buchanan, Proc. R.S.E., lvi. 108 ; R. C. Mossman, Proc. B.S.E. lvi. 108. 



