426 COERESPONDENCE. 



■ COHEESPONDENCE 



commdnioation from dr. robert hare, on a method of forming 



small weights. « 



August 29, 1856. 



I submit, in writing, tlie proposition respecting the production of 

 small weights which I made to the American Association for the 

 Advancement of Science, at a recent meeting. 



In chemical analysis, and in the assay of the precious metals, the 

 accuracy of the extremely minute weights employed is of the utmost 

 importance. 



As the governmont has undertaken to furnish standard weights 

 and measures for the larger operations or transactions of commerce 

 and the arts, without which accuracy and uniformity could not be 

 secured to the country at large, so it would seem consistent that to 

 the minute processes of the arts and sciences a help should be given 

 which otherwise seems not to be attainable. 



The usual process of making weights, by reducing them till they 

 exactly counterpoise a standard weight, cannot be pursued advan- 

 tageously when tbey are less each than a tenth of a grain. For 

 the making of weights below that size, measurement and division are 

 preferably employed. 



An instrument constructed by an ingenious and skilful machinist 

 (Tyler) is capable of dividing an inch into 1,400 parts by the action 

 of a ratchet and wheel, which may be so restricted in its motion as 

 only to move one tooth at a stroke, causing a platform to advance 

 only the fraction of an inch above mentioned. 



For producing by means of this instrument tenths and hundredths 

 of a grain, a convenient length of very fine palladium wire may be 

 employed. This being reduced in length very cautiously till it weighs 

 some equimultiple of a grain, a distance commensurate with the 

 length of this wire is marked upon a suitable narrow brass plate by a 

 knife. The number of the ratchet strokes which must be made in 

 order to measure this distance must be ascertained. 



Dividing this number by the number of grains will give the num- 

 ber of ratchet movements in a length of the wire equal in weight to 

 a grain ; again dividing this by ten will give the number of such 

 movements in a length equal to a tenth of a grain ; and, in like man- 

 ner, if divided by one hundred will give the number of the movements 

 in question requisite to designate the length equal to yoVo^ ^^^ ^ 

 grain. The length equal to as much of the wire as would weigh a 

 tenth of a grain being thus founds this distance is to be marked on 

 the brass plate with a sharp edge. 



A strip of steel is in th6 next place sharpened at each end to 

 a fine edge, and bent so as to resemble a long narrow staple, is 

 to be furnished midway with a scrc.v, by which the ends can be 



'-'■ Accidentally omitted in the last report. 



