PREFACE. 



Vll 



frozen fogs, and the great depth of snow on the summit in winter. The ice crystals 

 deposited by the fog prevent the use of any self-registering instruments, except inside 

 the Observatory, or outside in fine weather ; and in the latter case only if the instrument 

 be sufficiently portable to be taken in when fog comes on, when the temperature is below 

 freezing. 



Thus all records of temperature during winter on Ben Nevis are dependent on 

 personal observation ; and the method in use of hourly readings of thermometers in a 

 louvred screen, that can be at once removed when the louvres get choked, and replaced 

 by a duplicate outfit, is, in the opinion of the Directors, the only satisfactory way yet 

 devised of obtaining such a record. The screens are mounted on ladder-like stands, up 



Fig. 4. — the fort-william observatory. 



and down which they are moved as the depth of the snow requires. These stands are 

 shown in figs. 5 and 6. In the former they are coated with a moderate growth 

 of fog crystals, except where these have been scraped away to keep the screen clear ; 

 but in the latter, one of the two stands is entirely cleared, and the other covered with 

 mingled snow and ice, due to rain falling while the temperature is below freezing ; 

 to the left of the picture are the Observatory buildings, also covered with 

 the icy crust, except the chimney to the extreme left and the top of the 

 lightning conductor on the tower. These stands are about 12 feet high, but 

 frequently need to have an additional piece added to them before the winter is over, so 

 as to keep the screen at the proper height above the surface of the snow. The depth 



TRANS. ROY. S0C. BDIN. — VOL. XLII. b 



