INTRODUCTION. 3 



the adjustment of it to the ivory point, and the light shining through the upper part 

 of the tube shows clearly when the vernier has been brought down so as to just touch 

 the convex surface of the mercury that is to form a tangent to it. The cistern of the 

 barometer is 4407 feet above sea level, the height being taken from an Ordnance 

 Benchmark near the Observatory. A spare Fortin Barometer (No. 1252) hangs in a 

 similar case attached to the wall of the Observatory ; it was used as the Standard during 

 the greater part of the first year that the Observatory was open, 1st December 1883 

 to 14th November 1884, but since the latter date the present Standard (No. 1385) 

 has been continuously used. The last inspection of these barometers, in August 1901, 

 showed that they were in good order and the original Kew corrections still necessary 

 and sufficient. 



The barometric readings as printed in the tables are corrected for instrumental 

 error and reduced to 32° ; as explained above, the first figure of each entry is 

 omitted, and the highest and lowest values for each month put in bold-faced 

 type. The right hand column of each table contains the mean value for each day, 

 mean of the 24 hourly readings, and at the foot is given the mean of each hour 

 for the month. 



A Richard Aneroid Barograph is used as a check in the readings of the mercurial 

 barometer, but none of its records are printed. 



The Temperatures recorded at Ben Nevis Observatory are got from Dry and 

 Wet Bulb Thermometers in a louvred screen. The thermometers used have 

 degrees etched on the stem, and are mounted in glass scales with the bulbs pro- 

 jecting as least an inch beyond the scale. During the summer months the ther- 

 mometers are placed in an ordinary Stevenson Screen with their bulbs four feet 

 above the surface of the ground, which in the vicinity of the screen is all broken 

 rock without vegetation. During winter the thermometers are placed in a screen of 

 the Stevenson pattern but of slightly smaller size, which is attached to a ladder-like 

 stand, so that the screen can be raised or lowered as the depth of snow varies, and 

 the thermometers always kept between three and five feet above its surface. As 

 the snow sometimes lies from 10 to 12 feet deep, an adjustment of this sort is 

 absolutely necessary ; this pattern of screen is generally in use for about seven 

 months of the year. When the louvres of one of these screens get choked with 

 snow, it is removed bodily and a duplicate screen with fresh thermometers put out, 

 the former one being taken into the Observatory to be thawed and made ready 

 for use again when needed : all cases of such shifting of screens will be found noted 

 in the Log-Book. The thermometers used have all Kew certificates and the read- 

 ings as printed are corrected for instrumental error. The columns headed Max. 

 and Min. contain the highest and lowest hourly readings of the Dry Bulb for each 



