26 C. B. Comstock— Variation of a Zine Bar. 
Art. 1V.— Variation in Length of a Zine Bar - the same Tem- 
perature; by Gen. C. B. Comstoc 
{Communicated by Authority of the Chief of Engineers, U.S. A.] 
Tue U.S. Lake Survey possesses a steel] normal meter des- 
ignated as oe 1876), and a meter designated as (M. T. 1876), 
composed of a bar of steel and one of zinc so arranged as to 
form a metallic ronmhey tomnsea Both were made by Repsold. 
the essential parts are tubes o cast iron four meters long, each 
containing in its interior a steel and a zinc bar arranged to 
form a metallic thermometer. Irregularities in the results of 
comparisons of two bars in the same tube, which were very 
marked functions of the temperature changes, led to an exam- 
ination of the question whether a zine bar has always the same 
length at a given temperature. The results seem to show con- 
clusively that it has not. I have not met elsewhere with com- 
parisons establishing such a change; if they have been made, 
ese comparisons may give additional data. Mr. E.S. Wh eeler, 
who made the larger part of the comparisons, first called my 
attention to the indications of a set shown by the ordinary 
comparisons. 
As to the accuracy of the comparisons it may be said that 
they were made with an apparatus constructed by Repsold, in a 
comparing-room lined on all sides with saw-dust ; that this lin- 
ing reduces the diurnal temperature fluctuation to 0°83 F.; that 
the changes in the external mean daily temperature rarely pro- 
duce a change in the comparing- box exceeding 2°5 F. per day; 
that but two visits were made to the comparing- -room in a day; 
that the probable error in the result of one visit and compari- 
son of two steel bars one meter long is about 1"-9 (microns), 
and that artificial heat is not used. Temperatures were deter- 
mined by thermometers whose probable errors do not exceed 
0°05 i one lying on each meter = 
In the experiments with the zine bar of (M. T. 1876), this 
meter was alternately heated and cooled, and after each heating 
periments, its i gay varying in that time only about 3° 
F. In heating (M. T. 1876) it was taken from the ee 
room at a temperature of about 36° F. to another room 
kept at a temperature between 70° F. and 80° F. for realy 
rs or more, then it was replaced in the comparing-box, 
where it cooled slowly to the temperature of the comparing: 
