128 BOARD OF AGRICULTURE. 



less and so perfect, that we are hardly conscious that the process 

 is all the time going on around us ; and moi'e marvellous still, this 

 entire quantity is let down again upon the earth so gently that in- 

 jury is seldom done to the tenderest flower that blossoms upon its 

 surface. 



Inasmuch as evaporation depends upon temperature, it follows 

 that the quantity of vapor which rises from a given surface during 

 the warna months of the year, must be considerably greater than 

 the quantity that rises during the cold months. At London, the 

 amount of evaporation in the month of June is 3| inches ; the 

 amount in the month of January something less than half an inch. 

 I am not aware that the quantity is accurately known for our own 

 State, or for any special localit}'' of the State, for the entire year. 



It was determined for a few months at Orono last year. The 

 amount of evaporation at the State College, &c., from May 20th 

 to October 27th, extending over a period of a little more than five 

 months, was 18.54 inches ; and this was but a trifle less than the 

 rain-fall during that period; the latter being only 90-100 of an 

 inch in excess of the former. You will remember, however, that 

 in that time is included the month of October, when the quantity 

 of rain that fell in this State was nearl}^ three times the monthly 

 mean. The ordinary amount of rain-fall for a month in this State 

 does not vary much from three and a half inches. The amount 

 that fell in the month of October was very nearly ten inches. 



Some idea of the difierence in the amount of evaporation during 

 the different months of the year may be obtained from the numbers 

 which I propose to submit, indicating the pressure of vapor in the 

 atmosphere at Orono during the year 18G9 ; for the pressui'e de- 

 pends upon the amount, and this in turn upon the rapidity of 

 evaporation. When a large amount of vapor is in the air, it indi- 

 cates that the process of evaporation has been going on rapidly. 

 The pressure is indicated by the height of a column of mercury, 

 which the vapor is capable of sustaining. You are awai'e that 

 at the level of the sea, the mercury in the barometer stands on an 

 average at the height of thirty inches, or in other words, a column 

 of air extending to the height of forty-five or fifty miles is capable 

 of balancing a column of mercury biit thirty inches in lieight. 

 The pressure of the vapor of the atmosphere for the month of Jan- 

 uai-y, 1869, in the locality referred to, was 0.092 of an inch, that 

 is, the vapor in tlie air was capable of sustaining a column of mer- 

 cury 0.092 of an inch in height. For the month of February, 



