29 



Table XII showing loss of light in some typical Massachusetts 

 greenhouses running north and south. All results are of one test 

 made in winter. Outside light conditions in open air equal 100%. 



One layer of glass except where specified. 



It will be noticed that several sizes of glass were used in these 

 houses. In some cases the houses were double glazed either on 

 the side or roof, or both; i. e., besides the glass set in the top of the 

 sashbar there was another layer on the underside, with a space of 

 1 1-2 or 2 inches or more between them. These houses were all 

 about 18 or 20 feet wide and 80 to 200 feet long except No. 4, 

 which was 80 feet wide and about 300 feet long. No. 1 was an 

 old house with double glazed sides and poor light. No. 2 was 

 double glazed on both sides and roof. No. 3 was a new house with 

 fairly clean glass, while No. 4 was a large house of unusual width 

 and about 1.3 ^^ears old. No. 5 was glazed with different sized 

 glass which was so dirty as to exclude considerable light. No. 6 

 was double glazed both on the sides and roof, and Nos. 7 and 8 

 were new, modem houses. This series of houses is by no means 

 typical of modem greenhouses in this state except Nos. 7 and 8, 

 since most are glazed with smaller glass than that used at the present 

 day. Moreover, some of them were double glazed, an antiquated, 

 unscientific and impractical type of construction found only in 

 certain locations, and already referred to. 



The tests given here had a duration of two or three hours and 

 represent readings obtained from the glass alone; shadow casting 

 material being practically eliminated. The results show fairly well 

 the percentage of light excluded in this type of house, but to get 

 reliable averages it would be necessary to run experiments all day 

 long for a definite period, as the condensation of moisture occurring 

 between the two layers of glass varies from day to day. 



