i06 Prof. Bessel's Remarks on Mr. Hassler's plan for an 



out the direct light of the sun, they also rendered good ser- 

 vice, and were visible at great distances*. 



Mr. Hassler has also communicated his methods for the 

 comparison of the standard measures of length, and the re- 

 sults of their application ; we gain by this a new comparison 

 of the French and English measures, which I shall quote more 

 m particular. There were three meters present. One of 

 iron, which was one of those made by the committee of weights 

 and measures in Paris 1799, and distributed as authentic 

 among the foreign deputies ; the two others, the one brass, the 

 other iron, were Lenoir's, but not compared directly with the 

 original, they therefore were not considered as principal in the 

 results of comparison. These meters were compared with a 

 scale of Troughton, of eighty-two inches in length, divided 

 upon silver to tenths of inches, to which is added a microme- 

 tric apparatus to take off measures from the scale. Instead of 

 the usual method in comparing a meter a bouts with one a 

 traits, to place butting pieces with lines drawn near to the end 

 of them, the distances of which are measured by the micro- 

 scopes when these pieces are laid together, Mr. Hassler em- 

 ployed the end planes themselves; for that purpose he con- 

 structed the butting pieces exactly of the same thickness as 

 the meters, and obtained, by the close juxtaposition of both, 

 a line, which presented itself like a division line of the scale. 

 By means of several experiments (reduced to 32° Fahr. and 

 adopting the expansion of the iron and the brass, as Mr. Hass- 

 ler determined it by his own experiments, namely between the 

 point of melting ice and the boiling heat of water;) 



/iron = 0-0012534.363 



\ brass = 0-001 8916254. 



* To use the heliotrope, two conditions are indispensable ; the atten- 

 dance of an assistant at each signal station to direct it to the observer, and 

 its actual illumination by the rays of the sun. Had Mr. Hassler's operation 

 been intended to include no more than a net-work of great triangles, the 

 heliotrope might perhaps have been used, as no more than two signals need 

 have been observed from each station, and two assistants would have suf- 

 ficed for their management. But the survey being necc^sanly conducted 

 with a view to its inunediate application to geographical and hydrographi- 

 cal purposes, it would have been necessary to multiply the signals to such 

 an extent as to have rendered it impossible to employ so many separate 

 attendants. Mr. Hassler's signals also answer well even in a cloudy state of 

 the atmosphere, if the other circumstances be favourable, as frequently hap- 

 pens. The objection that two signals could rarely have shown an equally 

 well-defined image of the sun does not hold good, when a fixed instru- 

 ment observing without repetition is employed. We cannot therefore but 

 thuik, that for all general purposes the signals of Mr. Hassler are prefer- 

 able to the heliotrope of Gauss. 



the 



