184 EXPLOKATIONS ACKOSS THE GREAT BASIN OF UTAH. 



the regular development of this interesting phenomenon, may be found in the great 

 width and length of the valleys, which are free from any obstruction; in the large 

 quantity of heat radiated from their sparsely-covered and dry soil; in the powerful 

 fluctuations of the atmosphere, caused by the difference of temperature between the 

 bottom of the valleys and the upper regions of the air, and the great amplitude of the 

 daily oscillations of the temperature. Near Camp Floyd, in Cedar Valley, they may 

 be caused directly by the distribution and configuration of the mountains at the 

 northern end of the valley. The winds from the north and northwest, after sweeping 

 over the immense unbroken level of the Salt Lake Valley, when they approach Cedar 

 Valley, are divided into two branches by the mountains which separate Tuilla Valley 

 from the valley of Jordan River. The western branch meets, at the southern end of 

 Tuilla Valley, with the mountain mass of Floyd's Peak, and partly continues into 

 Rush Valley, partly is diverted to the southeast and enters the northwestern extremity 

 of Cedar Valley, across a depression in the O-quirrh Mountains. The eastern branch 

 enters Cedar Valley by various depressions in the much less elevated so-called Traverse 

 Mountains. These different currents, when meeting again, appear to form the whirls 

 whenever the accessory circumstances are favorable. 



HTGBOMETBICAL CONDITIONS OF THE ATMOSPHERE. 



I have already spoken of the smallness of the amount of rain, snow, and dew 

 which falls in Central and Western Utah. Before discussing this subject farther, I will 

 introduce some general remarks for the benefit of the scientific reader. 



The formation of vapor in the air is especially dependent upon two conditions, 

 namely, upon the temperature and upon the presence of water. With an unlimited supply 

 of moisture, vapor will be found in proportion to the height of the temperature; but with 

 equal degrees of temperature, more vapor will be formed in districts which abound in 

 water than in those which do not. Hence it follows that the absolute quantity of 

 vapor in the air, other circumstances being the same, is less in the interior of continents 

 than on the seashore. As more vapor is diffused through the air at a high temperature, 

 and as with an increasing heat the water evaporates more and more from the surface 

 of large masses of water and from the moist ground, the quantity of water contained 

 in the form of vapor in the lowest stratum of the air by which we art- surrounded will 

 diminish and increase in the course of the day. In climates of moderate humidity 

 such as Western Europe, the quantity of vapor in the air is generally increased as the 

 temperature rises with the rising of the sun. This, however, only lasts till about 9 

 a. m., when the ground becomes dryer, and an ascending current of air, occasioned by 

 the strong heating of the surface of the ground, carries the vapor on high, so that the 

 weight of water contained in the lower strata of the air diminishes, although the forma- 

 tion of vapor continues. This diminution continues till toward 4 p. m.; then the quan- 

 tity of water of the lower strata of the air again increases, because the upwardly-directed 

 current of air ceases to carry away the vapor formed. This increase lasts, however, 

 only until toward !) p. in., because the decreasing temperature puts a limit to the fur- 

 ther formation of vapor. In winter, when the action of the sun is less intense, the 

 state of the case is different. Then there is generally only one maximum of the quan- 



