44 



J. F. Tennant— 0« Shindard Weirjlits. 



[No. 1, 



The weight in standard air of a piece of brass whose weight in vacuo 

 is the same as that ot" P S., and whose density, conipared with that of water 

 at its maximum density (the brass being at the freezing point), is 8"14!30. 



If we know the value of a weight in terms of P S , we shall be able 

 to find its value in terms of W by adding the weight of air displaced by 

 the same weight of brass similar to that of which W is supposed to be made, 

 and deducting that actually displaced by the weight to be determined. 



The Standard Platinum-Iridium ounce sent me is certified to weigh (in 

 vacuo) 479'95979 grains in terms of P S., and the density has been assumed 

 as 21414, which is that of the 100 gramme weight. In English Standard 

 Air its weight is given as 480'00502 grains, but that datum is useless for 

 purposes of reference. It is called E I in the books of the Standards 

 Office in London, and I propose to retain this name. 



The ounce weight of the bullion set was certified to weigh 480'00145 

 grains in vacuo in terms of P S. and 480-00203 grains in English Standard 

 Air in terms of W. 



The following matter must be borne in mind in order that the 

 jDrocedure in my weighments may be understood : 



The sign = means that the weights on each side of it are equal in 

 vacuo. 



The sign = means that these are equal in air at the time ; and, in the case 

 of Commercial Weights, that they are sufliciently equal 

 for practical purposes at all times. 



The sign ^ means that the weights on each side being in the respective 

 pans of the balance there would be equilibrium. When no 

 division of the scale is mentioned as the resting point, it 

 is assumed to be 10 for Oertling No. 1 and 15 for Oertling 

 No. 2. 



On is one of the set of Gilt Bullion Weights — the subscript number denotes 



its nominal value in Troy ounces. 

 Pn is one of a set of grain weights which have been used for small quan- 

 tities, and n is the number of grains nominally : all weights 

 not less than 1 grain are of platinum and have been cleaned 

 by incandescence in a spirit-lamp. The tenths of grains 

 are of aluminum and the hundreths of uncertain material, 

 and are two riders (approxinately of one-tenth of a grain each) used 

 with the balance Oertling No. 1. 

 The Tables I have used in my reductions have been calculated by my- 

 self to the units of the Barometer and Thermometer scales commonly 

 used in England, and which it was most easy for me to refer to. That 

 for the density of air, has been calculated from the formula given by Pro- 

 fessor Miller, in his paper in the Philosojjhical Transactions, with the ueces- 



