64 



Mr. Gilchrist on the Balance 



(JULY 



the second, which virtually is the necessity of having " recourse to com- 

 putation," if this is to be considered an objection, what would become 

 of Science — more especially of the department of which Mr. T. is the 

 industrious cultivator ? A more serious objection to it is, that it blunts 

 the sensibility of the instrument, for, ceteris paribus, the lower the 

 centre of gravity is placed below the fulcrum, the less delicate is the 

 balance, and as this objection does not hold good with respect to the 

 specific gravity check, it appears to me the more eligible one. In the 

 weighing balance, the centre of gravity is placed below the fulcrum, to 

 produce oscillation; also horizontality of beam, when equal weights are 

 in the scale pans, but as oscillation is not required in the balance baro- 

 meter the first variety of beam, named by Mr. Taylor, maybe used, since 

 the least additional weight moves it, while it is prevented upsetting by 

 the dip or the immersion of the tube into or out of the mercury. In this 

 variety of balance, having also the points of suspension in the same 

 plane with the fulcrum, there is no tendency to move in any direction 

 when equality of weight exists, but remains equally at an angle with 

 thehorizon as parallel to it ; this therefore is exactly the balance wanted. 

 If I understand Mr. Taylor aright, he considers objectionable the quan- 

 tum of weight that will necessarily be on the balance, namely " tube 

 of iron," column of " mercury," &c. It is no doubt true that the more a 

 balance is loaded, it is the less delicate, but in the present case this ap- 

 pears more a theoretical than a practical objection, inasmuch as " in 

 Europe, where every obstacle in the shape of workmanship vanishes," 

 balances are made, which, when loaded much heavier than what the ba- 

 lance of the barometer requires to be, turn with much less than 7 grains* 

 A balance has been made of such nice sensibility, that, when loaded at 

 each end with 250 pounds, it turned with one grain.* 



As to the substance of which the tube consists, this necessarily re- 

 quires two qualities amongst others : 1st, impermeability to air; 2d, the 

 power of resisting the chemical action of mercury. Glass combines 

 both of these qualities, and the only objection to it is, its liability to frac- 

 ture when boiling the mercury— of the metals, only two, so far as I am 

 aware, are not acted on chemically by mercury — namely, iron and plati- 

 num. The former Mr. T. has shown to be objectionable, not being air 

 tight. How is the latter circumstanced in this respect ? If some of the 

 other metals are impermeable to air, would it not suit to plate with the 

 most eligible one so much of the tube as will not dip into the cistern ? 



I proceed to mention what appear improvements on the plan as 

 originally proposed — 1st. With respect to the register apparatus. — It 

 is advisable to remove the counterpoise weights UUU (Madras Journal 



* This balance was constructed on the plan and under the direction bf the late Captain 

 Kater, V.P.It.S. for verifying the national standard bushel— vide Phil. Trans. I32«. 



