266 REPORT OF STATE BOARD OF HORTICULTURE. 



Fifth — Mixing the air so as to prevent the cold air from sinking to the 

 surface. 



It is believed that all efficient methods of protection which have been 

 devised are embraced under one or more of these classes. The methods 

 will be grouped in accordance with the above classification. 



METHOD FOR RETARDING RADIATION. 



Since radiant heat is transmitted in straight lines, the erection of any 

 screen between the plants and the open sky tends to intercept the rays and 

 either reflect them back toward the earth or absorb them, thus raising the 

 temperature of the screen which checks the loss of heat, for the screen itself 

 now becomes a radiating surface and retuinis a portion of its heat to the 

 earth. Any substance which tends to obstruct the passage of heat rays forms 

 a more or less effective screen for checking radiation. 



Glass screens — In greenhouses and hotbeds advantage is taken of the 

 IJeculiar property of glass, which allows the heat rays of the sun to pass 

 through it, and is almost impervious to the dark heat rays from the earth 

 and plants. This is one of the most perfect screens possible, since it not 

 only prevents the loss of heat by radiation, but receives and retains the heat 

 from the sun. The expense precludes its adoption, except for the protection 

 of valuable plants and flowers. 



Screens of other solid materials have been quite extensively used in pro- 

 tecting vineyards and citrus gToves where intense cultivation is practiced, 

 and w^here the location of the groves, near an excellent market, admits of 

 profit even with expensive methods of cultivation. 



Clotli screens — In Italy and portions of France, scx'eens made of muslin 

 strung- on wires stretched on poles above the tops of trees or vines have been 

 used extensively. These screens are drawn on nights when frosts are jiroba- 

 ble, and pushed back during the day. When the season has advanced so as 

 to preclude further danger, they are taken down and stored. Of course, such 

 a plan could be operated only on a very limited scale, and would then be ex- 

 l)ensive. This i^lan has been recently successfully tried in the orange groves 

 of Southern California. 



Lath screens — During the past few years screens made of laths fastened to 

 ordinary telephone wire (the spaces between them being- about the width of 

 the laths) have been extensively used in Florida. These are spread over a 

 frame erected above the trees or plants. The screens serve not only as a fair 

 protection from frost, but also as a shade from hot sun. When no longer 

 needed, they can be rolled up and stored away for preservation. At first 

 thought it would seem improbable that a screen covering only half the space 

 (the spaces being as wide as the laths) would afford much protection ; but. 

 when it is considered that laths have considerable thickness, it is plain that, 

 while only one-half the vertical rays are screened, those inclined between 

 the vertical and horizontal are partially intercepted by the edges as well as 

 the faces of the laths. As a matter of fact, about three-fourths of the sky is 

 screened by this means. 



By placing the laths in north and south directions, the direct rays of the 

 morning sun are completely cut off from the orchard, which admits of' the 



