302 PROCEEDINGS OF THE AMERICAN ACADEMY 



der the action of a varying temperature, in order to decide how long 

 they must remain at nearly a constant temperature before comparisons 

 can be safely made. The comparing-room is a small triangular space 

 partitioned off from the cellar of the west wing of the Observatory. It 

 has two windows, one facing south and the other west. All the heat 

 of a furnace can be turned into the room through a pipe entering it 

 near the ceiling. The comparator is mounted on brick piers insulated 

 from the building. The temperature of the room can be considerably 

 reduced by means of two refrigerators, supported near the ceiling. 

 Centigrade thermometer No. 1 is imbedded in the base of the com- 

 parator, and packed with iron filings taken from the bedplate. Cen- 

 tigrade thermometer No. 2 is suspended by a fine wire about half-way 

 between the point where the heat from the furnace enters the room and 

 the upper surface of the comparator. 



Series I. consists of partial records of a comparison of a line-measure 

 steel yard by Troughton and Simms with a yard laid off on platinum- 

 iridium plugs inserted in the bed of the comparator. Series II. con- 

 sists of partial records of a comparison between two line-measure steel 

 bars, of which one is nickel-plated, the graduations being upon the 

 nickel surface. These two bars were made at the same time, and of 

 the same material. The bars have the following dimensions : — 



Length. Breadth. Depth. 



Nickel-plated bar, 40.4 in. 0.5 in. 2.0 in. 



Steel bar, 39.37 0.6 1.6 



A thin vertical lamina of platinum is inserted in the shorter bar. In 

 other respects, it differs from the longer bar only in its dimensions, and 

 in not being nickel-plated. 



After the bars were placed in position, they were not disturbed till 

 the close of the observations. Whatever changes took place, there- 

 fore, were due entirely to the action of temperature upon the bars. In 

 Series I. the value of one division of the micrometer employed was 

 .0000035 inch. In Series II. it was .0000197 inch, as stated on 

 page 291. 



Series I. 



DifT. in Length 

 Time. Thermometers. j M Divisions of 



h. m. No. 1. No. 2. Micrometer. 



1880, Mar. 22, 9 45 a.m. 5.7 — +35.6 



9 48 5.9 — -j-35.3 



9 50 6.4 — -j- 40 - 7 



Closed windows, and turned on heat. 



