§24 ON RADIANT HEAf» 



be speaking of plain tiles, or pantiles; but taking them to 

 be plain tiles, the least favourable supposition, and the 

 size of ours, a roof of 24 feet by 25, which would be 

 that of a house of middling size would take about 4000* 

 Now two thirds of a barrel of tar, at £2 6s. a barrel, the 

 highest price in the market at the present time, come to 

 £l 10s. 8d ; and the labour, at the rate of 6 men for 8 

 hours, the longest time in the two experiments above, at 5s. 

 a day, will be £l 4s; so that the whole additional cost of a 

 moderate sized roof would not exceed £2 15s. This must 

 very soon be reimbursed by the saving in repairs of the roof 

 alone; and all the inconvenience, beside the injury done to 

 the ceilings and goods, would be avoided. If coal tar were 

 used, v.hich, I should imagine would perfectly answer the 

 pir <»ose, supposing such a rcof to require a hundred weight, 

 this -low sells for 18s., so that the cost would be only two 

 guineas. C. 



XIII. 



Extract of a Letter from Prof. Kries, of Gotha, to Mr, 

 Gehlen, on Radiant Heat** 



JL Imagine a short historical note on radiant heat will not 



be unpleasing to your readers. 



Experiments The experiments of Pictet, made with two mirrors, in 



on radiant heat ^ f ocllg f , ie f which he placed a burning body, and 



century. thus set tire to combustible substances in the focus of the 



other, justly excited the attention of natural philosophers. 



Bu, it is surprising, that such experiments had been made 



more than a hundred years before, but were forgotten. 



Lambert, in his Pyrometry, says, on the authority of Zahn, 



that the experiment of collecting heat from a charcoal fire 



by a mirror of 18 inches diameter, and reflecting it to the 



distance of iO or 24 feet to a smaller minor of 9 inches. 



which so concentrated the rays, that tinder and matches were 



* Aim. de Chim. vol. LXXI, p. 158. Translated from Gehlen's 



Journal by Tassaert. 



kindled 



