OF ARTS AND SCIENCES. 



43 



on Merrimack River. Ward Hill, Mass. : Black clouds rising, " with 

 a swirling mass of light, smoky clouds along the whole length of the 

 upper edge ; at 16.45, the bank was overhead, stretching from N. E. to 

 S.W., and rushing on with great speed " ; about 17.00, a high S.W. 

 wind. Lowell, Mass. : Dark clouds rising in N.W. ; at 16.44, " a white, 

 foamy, frothy cloud, considerably disturbed, seemed to be pushed or 

 rolled rapidly along ahead of the black clouds, and came towards us fast 

 from the N.W. and N. " ; as it passed overhead, the wind changed 

 from S. to N. with sudden gusts at 16.45, and heavy rain at 16.49 ; a 

 relatively clear space was noticed between the squall-cloud and the 

 heavy black clouds that followed rapidly behind and above it. This 

 observer describes the squall-cloud as rolling along, the upper part 

 seeming to pass forward over the lower. Special attention is desired 

 on this point. Haverhill, Mass.: at 16.50, an immense windrow of 

 white clouds, extending from the W. to the N. horizon, came literally 

 rolling along toward our city ; behind it, a great area of black clouds 

 with continuous lightning ; very high, sudden wind, with moderate 

 rain, at 17.00. 



TABLE III. 



Change of Temperature in the Storm of July 9, 1885. 



A rapid fall of temperature is thus shown to accompany the storm- 

 front ; its average rate is probably a degree in one to three minutes, 

 but the most rapid change is much faster ; the total value of the change 

 ranges from ten to twenty or more degrees. The cause of the change 

 is to be found in part in the protection from hot sunshine by the storm- 

 clouds, but it is probably much more dependent on the low temper- 



