EXPOSURE TO THE .SUN. 95 



the use to which the houses .are put. For the vigorous winter- 

 growth of plants there can be no doubt that a nearly south- 

 east slope of the roof is most desirable. As was before 

 stated, but a very small percentage of the sun's light and heat 

 is intercepted or deflected when the ray impinges at nearly a 

 right angle. The loss continues to be trifling as the ray 

 departs from a perpendicular, and the angle with the plane of 

 the glass becomes more acute, and is only about four per 

 cent, when the angle is forty-five degrees with the glass. 

 But the loss increases at a rapid rate as the angle becomes 

 more acute. Hence it seems clear in theory, and it has been 

 found true in practice, that a roof, looking the sun full in the 

 face during the short days of winter, will produce the most 

 vigorous growth for all plants requiring light and heat. 

 Where only a moderate growth in winter is required, as, for 

 example, for the storage and propagation of bedding-plants, 

 which it is desirable to advance most rapidly in the spring 

 months, the north-and-south line for the ridge is most desira- 

 ble, since the morning and evening rays are very favorable, 

 and the mid-day rays, although considerably deflected, are 

 sufficiently strong for the desired purposes. But for forcing 

 work I am convinced that the roof should face southerly. 

 And hence I think it equally clear that the ridge-and-fur- 

 row style is not adapted to forcing. Only one-half of the 

 roof at best can face the south ; the other half is exposed to 

 the north winds, and cannot have the benefit of the sun. I 

 am told that in houses thus situated, and used for the growth 

 of lettuce, nearly double the time is required for maturing 

 the crop on the north bed that is necessary for the south 

 bed. 



We may then conclude that for most purposes for which 

 this style is suited, a north-and-south line for the ridge is 

 best, and that such houses are well suited for the propagation 

 and gradual advance of plants, also for the culture of all plants 

 which do not require the direct and strong rays of the sun. 

 It follows that the site for the houses is a plain, the ridge runs 

 as nearly as possible north and south, so that the morning and 

 evening sun falls equally upon both sides of the ridge. I 

 have no doubt that lettuce can be headed to perfection in such 

 houses, with much more certainty and at much less cost than 



