366 SCIENCE IN SHORT CHAPTERS. 



you syringe the stuff well from the inside, you make it 

 sufficiently clear to allow the air and light to pass through, 

 and it would probably last many years. He had tried the 

 experiment of dipping it in a very weak solution of tar, but 

 this had the effect of matting together the fine filaments, 

 so that it did not act so effectually as a strainer. It acted 

 best when wet, because the fine particles of soot adhered to 

 it, and moist weather was just the time when the greatest 

 quantity of soot fell. It might be easily tried in London 

 squares'to aid in the growth of flowers; he found that the 

 cabbage plants which were so protected throve remarkably 

 well, and he had no doubt that if flowers were planted and 

 a screen put over them until they were ready to bloom, it 

 would be a great advantage. The action of a little pent on 

 the top of a wall to protect fruit trees is very simple, and 

 the explanation was afforded by the experiments of Dr. 

 Wells on dew. The frosts which did the greatest mischief, 

 were due to radiation from the ground on clear nights ; 

 and it would be found that if one thermometer were placed 

 in a garden under an umbrella, and another on the open 

 ground near it, the differences of temperature would be 

 very considerable ; on cloudy nights there was very little 

 difference. Last night there was only a difference of 2, 

 but a few nights before it was 6. The period of greatest 

 cold might not probably be more than hour, but it would 

 be sufficient to do a great deal of mischief, and anything 

 which would check the radiation would have the required 

 effect. In the case of loosening the fruit trees from the 

 wall there was, probably, a double action ; it prevented the 

 tree being forced on by the warmth or the wall in the day- 

 time, and also avoided the chilling effect at night, a rough 

 wall being a good radiator, and sinking to a low tempera- 

 ture. Ho did not think there was much danger to be 

 apprehended from wind, because the canvas being so open, 

 the wind would pass freely through it ; but he had not seen 

 it subjected to any violent gale. 



