no HISTOHY OF 



111 this manner, the sea supplies sufficient humidity to the air, 

 for furnishing tlie earth with all necessary moisture- One j)ar£ 

 of its vapours falls upon its own bosom, before it airives upon 

 land. Another part is arrested' by the sides of mountains, and 

 is compelled, by the rising stream of air, to mount upward 

 towards the summits. Here it is presently precipitated, 

 dripping down by the crannies of the stone. In some places, 

 entering into the caverns of the mountain, it gathers in those 

 receptacles, which being once filled, all the rest overflows ; and 

 breaking out by the sides of the hills, forms single springs. 

 Many of these run down by the valleys or guts between the 

 ridges of the mountain, and, coming to unite, forin little rivulets 

 or brooks ; many of these meeting in one common valley, and 

 gaining the plain ground, being grown less rapid, become a river ; 



A set of experiments upon the evaporation from ground covered with 

 vegetables, aud from bare soil, was made by Mr Thomas Hoyle and Mr Ual- 

 toii, at Manchester, during the years 1796, 1797, 1793. They got a cyliodri- 

 cal vessel of tinned iron, ten inches in diameter, and three feet deep. There 

 were inserted into it two pipes turned downwards for the water to run 

 off from it into bottU's. One of these pipes was near the bottom of the vessel, 

 the other was an inch from the top. This vessel was filled up for a few 

 inches with gravel and sand, and all the rest of it with good fresh soil. It 

 was then put into a hole in the ground, and the space around filled up with 

 earth, except on one side for the convenience of putting bottles to the two 

 pipes. Water was poured on to sadden the earth, and as much as would was 

 suffered to run through without notice, by which the earth might be consi. 

 dered saturated as with water. For some weeks the soil was constantly above 

 the level of the upper pipe, but latterly it was always a little below it ; 

 which made it impossible for any water to run through the upper pipe. For 

 the first year the soil at top was bare, but during the last two years it was 

 covered with grass the same as a green field. Things being thus circum. 

 vtanced, a regular register was kept of the quantity of rain water that ran 

 off from the surface of the earth by the upper pipe (while that took place,) 

 and also of the quantity which sunk down through the three feet of eartli, 

 and ran out through the lower pipe. A rain guage of the same diameter 

 was kept close by to find the quantity of rain for any corresponding time. By 

 this apparatus the quantity evaporated from the earth in the vessel during 

 three years was ascertained. The annual evaporation was 25.1.58 inches. 

 Now if to the rain we add five inches for dew (not reckoned in Mr Dalton's 

 observations), it tollows that the mean annual evaporation from earth at 

 Manchester, amounts to thirty inches. It follows likewise, from these ob- 

 servations of Dalton and Hoyle, that there is but little difference between the 

 evaporation o! green soil and bare soil. For the evaporation during the firs! 

 year, when the soil in the vessel was bare, differed but little from that of th* 

 two I'jilowing years when i^vas covered Vi'ith grass. 



