LABORS OF THE JOINT COMMITTEE. 153 



form on the south or south west side of the parent cloud, 

 where the dew point is generally highest in this latitude 1 

 and, in this case, does the rain travel in the same direction 

 as in the other ? 



If it should be found that some rains travel towards the 

 south or south west, in consequence of the cold air, through 

 which rain has passed, pressing in that direction, and the 

 warm air, with a high dew point, coming from the south- 

 west, glancing up over it, as over a mountain ; how are 

 these rains to be distinguished from the others? Does the 

 rain vary in its direction and velocity along the surface of 

 the earth with the different seasons of the year? and is 

 there any general law on this subject? 



It would be highly useful to note the time when these 

 columnar clouds begin to form in the morning, and when 

 they disappear in the evening ; and let those who live near 

 a mountain test the rule given above for finding the height 

 of the base of these clouds, by noticing how the complement 

 of the dew point increases in the morning, as the clouds rise 

 higher and higher up the side of the mountain. 



122. On this point the following plan maybe adopted; 

 Let the observer previously take his barometer with him up 

 the side of the mountain, and mark how much it falls at 

 particular places, which can be distinguished from the 

 valley below, where the observer lives ; and then let him 

 observe whether the complement of the dew point is four 

 degrees for every inch of mercury that the barometer 

 would fall on being elevated to the base of the cloud. By 

 a great number of such observations, carefully made, the 

 rule given above may be corrected if it should be found 

 not exactly accurate. 



123. It will also be very important to note the direction 

 of a very lofty cloud (which seems to be formed out of the 

 top of a rain cloud,) very thin and fibrous, called in the 



20 



