136 PHYSICAL GEOGRAPHY. 



snow-fall is very slight during autumn and winter, the snow-cloud being 

 below the summit ; but in spring, when showers become frequent in the 

 valleys, there are invariably heavy falls of snow on the mountain. During 

 a thunder-storm in April, when the thunder could be heard and the light- 

 ning seen, we were having one of the thickest snow-storms of the season. 

 Nearly all the optical phenomena seen elsewhere on mountain summits 

 have been observed on Mt. Washington. Rainbows, with three supernu- 

 merary bows, have been seen for hours on the clouds ; coronas, of large 

 and small dimensions ; anthelia or glories of light, the prismatic circles 

 surrounding the shadow cast far out on the clouds ; halos, and parhelia. 

 The spectre of the Brocken, though rare, was seen by Mr. S. A. Nelson. 



DIAGRAMS. 



Diagram I shows the fluctuations in the annual rain-fall in the Atlantic 

 states, Maine to Maryland, from 1805 to 1867. From the fluctuations 

 as shown in this diagram, there are groups of years of unusual amount 

 of rain, followed by groups of years of drouth ; and, on the whole, it indi- 

 cates an increase of rain. The figures on the left are the per-centage 

 of the mean amount. 



Diagram II shows the fluctuations in the annual rain-fall in the upper 

 Connecticut valley, from observations taken at Lunenburg, Vt. This 

 shows similar groups of years. An unusual amount of rain-fall does not 

 necessarily imply that it was distributed throughout the year, so that 

 there was no drouth in summer; for, while the amount of rain in 1871 

 was above the average, yet the summer of that year was regarded as very 

 dry. 



Diagram III shows the fluctuations in the annual snow-fall at the same 

 locality, and by the same observer, as in Diagram II. The fluctuation, 

 however, is greater than in the rain-fall; for the greatest amount, 167.5 

 inches, is more than twice as much as the mean, 83.1 inches, and the 

 least amount, 41 inches, is less than half the mean; yet there are similar 

 groups of years, though at no time does it show more than three consec- 

 utive years, when the amount was greater than the mean. 



Diagram IV shows the annual fluctuations in rain-fall at Lake Village 

 from 1857 to 1873. The observations were taken under direction of the 

 Lake Company. 



