230 Translation from the AstronomicalJour. of Hamburgh. 



ever Mr. H. says, that even without the direct Hght of the 

 sun, they also rendered good service, and were visible at 

 great distances.* 



Mr. H. has also communicated his methods for the com- 

 parison of the standard measures of length, and the results 

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

 the French and English measures, which I shall quote more 

 in 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 au- 

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

 brass, the other iron, were of Lenoir, but not compared di- 

 rectly 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 micrometric 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 microscopes when these pieces are laid 

 together, Mr. H. employed the end planes themselves, for 

 that purpose he constructed 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° Fah. and adopting the expansion of the 

 iron and the brass, as Mr. H. determined it by his own exper- 

 iments, namely between the point of melting ice and the 

 boihng heat of water ;) 



* To use the heliotrope, two conditions are indispensable: the attendance of 

 an assistant at each signal station fo direct it to the observer, and its actual il- 

 lumination 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 per- 

 haps have been used, as no more than two signals need have been observed 

 from each station, and two assistants would have sufficed for their management. 

 But the survey being necessarily conducted with a view to its immediate ap- 

 plication to geographical and hydrographichal purposes, it would have been ne- 

 cessary to multiply the signals to such an extent as to have rendered it impos- 

 sible 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 

 favorable, as frequently happens. The objection that two signals could rarely 

 have shewn an equally well defined imago of the sun, does not hold good, 

 when a fixed instrument observing without repetition is employed. We can- 

 not therefore but think, that for all general purposes, the signals of Mr. Hass- 

 ler are preferable to the heliotrope of Gauss. 



