LABORS OF THE JOINT COMMITTEE. 149 



a self-registering thermometer, and a rain-guage, have been 

 purchased for each county in the State, which are to be 

 placed in the hands of some skilful observer, who shall vol- 

 unteer to keep a journal of the weather according to a 

 common form prescribed by the committee. 



My principal object in writing these hints is, not merely 

 to assist our correspondents in the State of Pennsylvania, 

 but to induce many others to keep journals of the weather 

 according to their means, and thus contribute their mite 

 towards enabling the committee to find out the course that 

 storms take over the surface of the earth in all the different 

 seasons of the year, and, consequently, their velocity and 

 shape, with the force and direction of the wind in their bor- 

 ders, both at the surface of the earth, and in the region of 

 the clouds. Now, as these primary and highly important 

 observations may be made without instruments, I beg that 

 no gentleman to whom this circular is sent, may be deterred 

 from keeping a journal of the weather from an idea that his 

 observations can be of but little avail. 



If he faithfully records all the phenomena of the weather, 

 particularly of the winds, clouds, and rains, it will be of in- 

 calculable advantage in the further investigation of storms. 



119. To those who have a thermometer, it will be inter- 

 esting to know that the height of the base of those clouds 

 (which generally form in a summer day, when the heavens 

 are not overspread with clouds, and disappear in the night; 

 and which, when large and well formed, have broad, flat, 

 dark bases, with tops as white as snow, and rising some- 

 times to a great height at the top, while the base continues 

 on the same level, assuming the appearance of a cone) can 

 be ascertained in the two following ways : 



1st. Find the dew point in the manner directed by the com- 

 mittee in their printed instructions (98), and the difference 

 between this and the temperature of the air at that time is 

 called the complement of the dew point. 



