GENERAL PRINCIPLES. 



129 



larger per-centage. The following experi- 

 ment was tried on four bars, cast in the 

 form of a double-faced railway rail, 1^ 

 inches deep, \ an inch wide at top and bot- 

 tom, and \ of an inch at the centre. The 

 length of each bar was 18 inches, with 15 

 inches between the supports. No. 1 was 

 cast in green sand ; No. 2 in dry sand ; 

 No. 3 cast in a chill • and No. 4 in a 

 chill, and afterwards annealed. 



No. 1 weighed 32.5 oz., bore 1232 lb., and 



deflected 0.130 inch. 

 No. 2 weighed 30.5 oz., bore 1006 lb., and 



deflected 0.114 inch. 

 No. 3 weighed 34.75 oz., bore 784 lb., and 



deflected 0.053 inch. 

 No. 4 weighed 34.5 oz., bore 2520 lb., and 



deflected 0.140 inch. 



The advantage in favour of cast-iron, 

 treated like No. 4, is evidently little less 

 than one hundred per cent over No. 1, 

 and three hundred per cent over No. 3. 



With such improvements in lessening 

 the quantity of metallic surface and add- 

 ing so much to its strength, we think 

 metallic roofs are now placed in a different 

 position from that which they formerly 

 occupied; while ventilation is supplied 

 both more abundantly, and upon more 

 correct principles. A deficiency of ven- 

 tilation no doubt long tended to increase 

 the prejudice, if so it may be called, 

 against metallic roofs. 



In the erection of plant structures, 

 the error of carrying them too high 

 ought to be avoided. To suppose that 

 we can produce head room for many of 

 the palms and other tropical trees to 

 develop themselves fully in is truly 

 ridiculous. The very circumstance of 

 giving increased height to such houses 

 has the tendency to draw up the plants 

 to an unnatural degree, quite out of all 

 proportion to their other parts. Nor 

 can the observer see these plants to 

 advantage confined within the narrow 

 circumference afforded even in the largest 

 houses hitherto erected. Whoever wishes 

 to examine the beauties of a well-grown 

 timber tree would certainly not fix the 

 spot of observation close to the trunk ; 

 nor can the grandeur of a forest be 

 appreciated by wandering amongst the 

 naked stems. 



All extra lofty houses hitherto erected 

 have been most unsatisfactory • and as 

 the great majority of flowering plants 



VOL. I. 



are of no extraordinary altitude, why 

 should we attempt to accommodate a 

 few at the expense of the many? — why 

 accommodate the loftier palms, and still 

 loftier Araucarias, which rarely, if ever, 

 reward us by a sight of their insignifi- 

 cant flowers, and neglect the thousands of 

 moderate-sized shrubs and plants which 

 exhilarate and surprise us by the splen- 

 dour and perfume of their gorgeous 

 blossoms % 



There is no great difficulty in the 

 erecting of these very lofty houses ; but 

 there is in the management of them 

 afterwards, both in regard to ventilation 

 and heating. The large domical conser- 

 vatory which at one time graced Bretton 

 Hall, and was then the wonder of the 

 age, and which was erected at a cost of 

 somewhere about .£10,000, was defective 

 in a very great degree as regards the 

 culture of the plants within it. We are 

 informed by Mr Marnock — who was 

 gardener at Bretton Hall at the time of 

 its existence — in his excellent Journal 

 for 1845, p. 633, that it was 60 feet 

 high, and of a conical form. In the 

 management of the temperature, he 

 says, " During mild weather all went on 

 very well, and especially in summer, when 

 the external air could be admitted into 

 the house with impunity, and without 

 risk to the plants ; but throughout the 

 winter, and especially when the external 

 temperature was low, and the winds 

 frosty and boisterous, then arose the 

 difficulty, and mischief to the plants was 

 unavoidable. To explain this — to those 

 at least who are at all conversant with 

 the nature and properties of heat — it will 

 only be necessary to remark, that, after 

 all that can be allowed for or said in 

 behalf of radiated heat in maintaining 

 the temperature of a hothouse on a level 

 with the pipes and the floor, the practical 

 effect is not to be mistaken ; the heated, 

 and of course rarified, particles of air, 

 as they come in contact with thej hot- 

 water pipes — or, as in the case to which 

 we refer, steam pipes — rush upwards, in 

 order to give place to the colder and 

 denser particles ; and as heat is gene- 

 rated below, it ascends as rapidly 

 upwards, till the upper part of the 

 lofty structure is heated to excess. In 

 the large house to which we refer, the 

 difference of temperature in ordinary 



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