Mr. Patten's Air Pump, Gazometer, (5^c. 97 



poses, but not for weighing the gases and taking accurately 

 specific gravities ; and in analysis where the weights are 

 often repeated it amounts to considerable. The only ac- 

 curate method is to make the weight itself the standard. (I 

 believe a French proposition.) To do this is the object 

 of the beam ABC, (fig. Gth.) made of steel, suffi- 

 ciently strong but light, the dish is suspended at A, the 

 beam itself upon an axis at B ; at C is the milled head of 

 a long screw that is fitted with a shoulder and axis, and 

 goes through the slide E that traverses upon BC and car- 

 ries the weight D. Now suppose it is wished to obtain 

 10 grains,- place that weight in the dish F, and screw 

 back the weight D until it exactly counterbalances it — if the 

 weight be now removed and a quantity of the substance to 

 be weighed be substituted until the index points to where 

 it did at first, there will then be very nearly the exact 

 weight with but small allowance for friction — for were this 

 beam a common one, and so nicely constructed as to turn 

 with the 100th part of a grain, it would by making the 

 distance from AB four" times greater than from B to D 

 the point of suspension of the weight — turn with the 400th 

 part of a grain. It is apparently an objection that 100 

 grains at A will require 400 at D, but the fact is settled by 

 Coulomb that this kind of friction does not increase in an 

 equal proportion with the weights used, that is, if with a 

 pound in each scale, a beam turn with one grain, if there 

 were two pounds in each, it would not require two grains. 



This beam may be used as a steel yard by screwing the 

 weight D to any number marked upon the scale, and 

 should a greater quantity be required than that marked in 

 the first line, another weight double of D may be substi- 

 tuted. These instruments are a few of a number I had 

 occasion for while engaged in studies to which I was once 

 particularly attached, but as they are in a great measure in- 

 compatible with my professional occupation, I am compel- 

 led in a great measure with reluctance to abandon them ; 

 when leisure and disposition again combine, I will send 

 you, should you think they may he of any use to the schol- 

 ar or the analyst, some further communications, 

 very respectfully, 



your obd't. ser't. 



JOSEPH H. PATTEN. 



Vol. IX.— No. I. 13 



