INTRODUCTION. 



year onwards the records are printed here. Eye observations are taken seven times a 

 day, and hourly values are obtained for the Barometric Pressure, Dry and Wet Bulb 

 Temperature, Rainfall and Sunshine from self-recording instruments. These hourly 

 values are printed side by side with the corresponding data from Ben Nevis 

 Observatory : on subsequent pages in each year will be found the Eye readings of 

 temperature and amount of cloud. 



Barometer. — Tn the photographic barograph from which the hourly values are 

 obtained an image of the top of the mercury column is focussed in a drum covered with 

 sensitised paper. The drum is turned by a clock, which also interposes a screen every 

 two hours for a few minutes, thus marking the time by making a gap in the 

 photographic trace. Intermediate times can then be measured on the curve from these 

 gaps by a scale. The trace when developed is placed in a frame and measured by a 

 scale fitted with a vernier reading to thousandths of an inch. The barograph has a 

 temperature compensation which corrects for changes in the temperature of the mercury 

 column. The instrumental correction of the trace is determined for each day by 

 readings of a Fortin barometer which hangs alongside the barograph. The cistern of 

 this barometer is 42 feet above sea level. In the printed tables the readings of the 

 Fort-William barograph are reduced to 32° and sea level : as explained above, the first 

 figure (either a 2 or a 3) of each entry is omitted, and the highest and lowest values 

 in each month are put in bold-faced type. 



The Temperatures at Fort- William Observatory are obtained from the traces of 

 photographic Dry and Wet Bulb Thermometers. These thermometers are placed in 

 the North wall of the Observatory, the bulbs being outside in a large louvred screen, 

 while the stems pass through the wall, and the fluctuations of the mercury columns 

 are photographically recorded in a similar manner to the barograph. These bulbs 

 are five feet above the grass-covered surface of the ground. The louvred screen also 

 contains Dry and Wet Bulb Thermometers for Eye reading, which are read seven times 

 a day, and a Maximum and Minimum, which are read and set at 22 h (10 p.m.). The 

 photographic trace when developed is placed under a glass scale and the temperatures 

 at each hour read off : these are the values printed here. The Eye readings are used to 

 control the scale values of the photographic trace, but are not published. The arrange- 

 ment of the printed tables is the same as that of the Ben Nevis Observatory Dry 

 and Wet Bulb, except that the Max. and Min. columns for Fort- William give the 

 highest and lowest points of the trace for each day, instead of the highest and lowest 

 hourly readings. 



Towards the end of each year's tables will be found the readings of Temperature 

 in a Stevenson Screen and the Amount of Cloud at Fort-William Observatory. This 

 screen stands on a grass plot to the south of the Observatory and is freely exposed to 



