286 SCIENCE HT SHORT CHAPTERS. 



The size of the quartering proposed is 2| by 1 inch, which, laid 

 edgewise, would bear the weight of a man on a plank while nail ing 

 down the canvas. The canvas has a stout cord-like edge or selvage 

 that holds the nails well. 



I find that what are called " French tacks"' are well suited for nail- 

 ing it down. They are made of wire well pointed, have good-sized 

 Hat clout heads, and are very cheap. They are incomparably superior 

 to the ordinary rubbish sold as " tin tacks" or " cut tacks." The 

 construction of such a conservatory is so simple that any industrious 

 artisan or clerk with any mechanical ingenuity could, with the aid 

 of a boy, do it all himself. No special skill is required for any part 

 of the work, and no other tools than a rule, a saw, and a hammer. 

 >Side posts and stronger end rails would in seme cases be demanded. 



I have not been able to fairly carry out this project, inasmuch as I 

 reside at Twickenham, beyond the reach of the black showers of 

 London soot. I have, however, made some investigations relative to 

 the climate which results from such inclosure. 



This was done by covering a small skeleton frame with the canvas, 

 putting it upon the ground over some cabbage plants, etc., and plac- 

 ing registering thermometers on the ground inside, and in similar 

 position outside the frame ; also by removing the glass cover of a 

 cucumber frame, and replacing it by a frame on which the canvas is 

 stretched. 



I planted 300 cabbages in November last, in rows on the open 

 ground, and placed the canvas-covered frame over 18 of them. At 

 the present date, March loth, only 26 of the 282 outside plants are 

 visible above ground. All the rest have been cut off by the severe 

 frost. Under the frame all are flourishing. 



I find that the difference between the maximum and the minimum 

 temperatures varies with the condition of the sky. In cloudy 

 weather, the difference between the inside and the outside rarely 

 exceeds 2 Fahr., and occasionally there is no difference. In clear 

 weather the difference is considerable. During the day the outside 

 thermometer registers from four or five to seven or eight degrees 

 above that within the screen during the sunshine. At night the 

 minimum thermometers show a difference which in one case reached 

 14 3 i.e., between 23d and 24th February, when the lowest tempera- 

 ture I have observed was reached. The outside thermometer then 

 fell to 8 Fahr., the inside to 22. On the night of the 24th and 25th 

 they registered 15^ oiitside, 25^ inside. On other, or ordinary 

 clear frosty nights, with E. and N. and N. E. winds, the difference 

 has ranged between 4 and 6, usually within a fraction of the aver- 

 age, 5. 



The uniformity of this during the recent bright frosty nights, fol- 

 lowed by warm sunny days, has been very remarkable, so much so 

 that I think I may venture to state that 5 may be expected as the 

 general protecting effect of a covering of such canvas from the mis- 

 chievous action of our spring frosts which are due to nocturnal radia- 

 tion into free space. Thus we obtain a climate, the mean of which 

 would be about the same as outside, but subject to far less variation. 

 How will this affect the growth of plants desirable to cultivate in the 

 proposed canvas conservatories ? 



In the first place, we must not expect the results obtainable under 

 glass, which by freely transmitting the bright solar rays, and absorb- 



