Translation from the Astronomical Jour, of Hamburgh. 233 



way, as long as the toise itself is yet obtainable as easily as 

 it was at the time of the construction of the meter. 



The apparatus which Mr. H. had constructed for the meas- 

 urement of the base line, differs essentially from all that are 

 known to me ; therefore I will describe it somewhat more 

 particularly. The ends of the bars are not planes, but cut 



out, so that viewed from above they present the form S, 



over this middle excavation the hair of a spider's web is 

 stretched, which therefore indicates the end of the bar : 

 over each of the ends a compound microscope is placed, 

 which stands upon a separate support, and therefore does not 

 change its place when the bar is moved or taken away. 

 When this microscope is placed over the spider's web, the 

 place of the end of the bar is determined by it ; the bar can 

 then be taken away, and the other end of it can be made to 

 coincide with the point where the first had been before seen 

 to coincide with the cross strokes of the microscope, which 

 in the mean time has retained its position independently. 

 The microscope has the following arrangement : the object 

 glass consists of two half lenses of different foci, one of 

 which makes, in the focus of the eye glass, an image of the 

 spider's web of the bar, and the other an image of two rec- 

 tangular crossing black lines, drawn upon an ivory plate, 

 which is fastened to the microscope : this arrangement can 

 be elevated and lowered, and moved in two horizontal direc- 

 tions at right angles to one another. In the use, the stand 

 being first properly placed, the microscope is brought to that 

 elevation in which the spider's web thread is distinctly visi- 

 ble, then it is moved until this thread appears exactly to cut 

 the cross upon the ivory plate ; the bar is then removed and 

 advanced one length forwards, the end of it is next brought 

 into the proper position by the mechanism of the bar, and it 

 is moved by it until the spider's web of this other end coin- 

 cides again by an optical contact with the cross on the ivory 

 plate. Of these microscopes there are three with all their 

 arrangements ; the last ones always remain standing during 

 the next subsequent operation, that in case of any accident 

 the work might be begun again from them. The bar itself 

 is a junction of four pieces, each of two meters in length, 

 held together by iron clamps : the inclination of this bar to 

 the horizon is measured by a sector, nearly as in Delambre's 

 apparatus. When the work is interrupted during the night, 

 Vol. XVI.— No. 2. 3 



