No. 3. 



Lightning Rods. 



97 



Its strong recommendation consists in the 

 small compass of bulk and weight which 

 renders cheap and easy, the transportation 

 to any distance, of a great amount of fertil- 

 izing power. A calculation has been made 

 by a careful and discreet farmer near Peters- 

 burg, to show that guano, at the rate of even 

 four liundred pounds to the acre, which is a 

 very liberal allowance, two to tiiree cwt. 

 being the usual quantity, does not cost, at 

 $3 per hundred, more than half as much as 

 the stable manure required to produce the 

 same results, at- twenty-five cents for the 

 two-horse wagon load, when hauled from 

 one and a half to three miles. 



To gratify our agricultural readers, we 

 subjoin the analysis of the genuine guano 

 by Prof. Ure, of London. So great is the 

 demand for Peruvian guano, that a spurious 

 compound in imitation of it has been manu- 

 factured. 



Average result of analysis of the genuine 

 guano, in reference to its agricultural value, 

 by Professor Ure, M. D. F. R. S. 



Azotized organic matter, included nrate of 

 ammonia, and capable of atTording from 

 8 to 17 per cent, of ammonia by slow de- 

 composition in the soil. 50.0 



Water 11.0 



Phospjiate of lime 25.0 



Ammonia, phosphate of magnesia, phos- 

 phate of ammonia and oxalate of ammo- 

 nia, conlaiiiiiig from 4 to 9 per cent, of 

 ammonia 13.0 



Siliceous matter from the crops of birds, 1.0 



100.0 



To distinguish " the true from the sham," 

 Professor Ure says that genuine guano, 

 when burned upon a red hot shovel, leaves 

 a white ash of phosphate of lime and mag 

 nesia, whereas the foreign substance leaves 

 a black fused mass of sea salt, copperas, a,nd 

 sand. The specific gravity of good fresh 

 guano is never more than 165, water being 

 100, whereas that of this substance is as 

 high as 217, as produced by the sand, salt, 

 and copperas in it. — Exchange Paper. 



From the Saturday American. 

 liightuing: Rods. 



I WILL consider the several queries pro- 

 po.=:ed in your favour of August 30th, in 

 order. 



1. " Do the square rods possess any supe- 

 riority over the round ones as conductoi-s of 

 electricity 1" 



Faraday, one of the best authorities on 

 electricity, asserts that the conducting power 

 of a rod is proportioned to its mass, or quan 

 tity of matter and not to its surface; and the 

 same doctrine is assumed by an English 

 writer, in "Sturgeon's Annals of Electrici 



ty." It is admitted that the fluid pervades 

 only the surface ; still it is maintained that 

 the conducting power depends on the mass. 

 I have not in my possession any set of ex- 

 periments which authorises the opinion of 

 Faraday, but he doubtless had, or he would 

 not have asserted it to be a lact. If this 

 doctrine be true, then so long as the mass 

 is the same, it is of no consequence whether 

 the rod is square or round. 



2. " Are a number of points attached to 

 the extremity of a rod preferable to one?''' 



According to the experiments of Earl 

 Stanhope, made more than sixty years ago, 

 a single needle will discharge a Jeyden jar 

 more rapidly than a bundle of the same ; 

 and in conformity with this, a single point 

 is generally used for the termination of 

 lightning rods in Europe, and I believe one 

 is preferable to several — especially where 

 they diverge from each other as they com- 

 monly do. In the lightning rods constructed 

 by a skilful manufacturer in this city, the 

 upper termination is formed of a sharp cop- 

 per spindle, at the base, of the same diame- 

 ter as the rod — say an inch or three-fourths 

 of an inch. This is permanently gilded by 

 the electrotype process. It makes a beauti- 

 ful finish; as may be seen in the rods re- 

 cently erected on the new Library of Yale 

 College. The joint where the spindle is 

 attached to the rod is as perfect as possible — 

 a cylindrical projection on the spindle, nicely 

 turned, fitting closely into a hole in the end 

 of the rod, nicely bored. All the parts of a 

 rod should be fitted by joints as close as this. 

 Many rods have proved defective, within my 

 knowledge, merely because the parts were 

 united loosely by the hook-and-eye joint. 



3. " Are tubes to be preferred to solid 

 cylinders, on account of the greater surface 

 exposed 1" 



If Farady's doctrine that the conducting 

 power is proportioned to the mass, is true, 

 then the tubes, of a given diameter, cannot 

 be equal in power to solid rods. 



4. "Do the projecting points placed along 

 the whole length of a rod, and at right-angles 

 with it, in some forms of conductors, increase 

 the efficacy of the rods ]" 



In case a rod is well connected with the 

 ground, so as to deliver the charge freely, I 

 think such points are unnecessary, so far as 

 they are supposed to dissipate the charge, 

 and therefore to render it less dangerous 

 than when it traverses the rod in a concen- 

 trated state. They are sometimes appended 

 on the idea of inviting cr directing a lateral 

 or horizontal charne ; but I have never .seen 

 any well attested fact of their utility in such 

 cases. It appears to me of great importance 

 to preserve all possible simplicity and cheap- 



