290 Notices respecting New Books. 



each is so adjusted as to be in the outer focus of its object-glass. 

 The microscopes revolve on the axis of the telescopic microscope in 

 a tube fastened to a horizontal plate attached to a tripod-stand with 

 levelling-screws, and furnished with longitudinal and lateral adjusting 

 screws. On one side, secured to the brass bar, is the spirit-level for 

 levelling the microscopes ; and on the other, firmly attached to the 

 centres of the bars by a brass plate, is a telescope embraced by a 

 brass collar with a small cylinder jirojecting from one side, which 

 turns in a socket attached to the plate : thus affording it a vertical 



motion, allowing objects to be seen in opposite directions The 



compensation microscopes are seven in number ; the weight of each 

 microscope is 5 lbs." — P. 11. 



The mode of using the apparatus will be readily apprehended from 

 the above desciiption. When the several sets of bars and microscopes 

 are properly adjusted in the line of the base, the microscopes stand 

 vertically over the tongues at the adjacent ends of each two sets of 

 bars, and the dots on the tongues are respectively bisected by the 

 wires of the two outer microscopes, so that the distance between 

 the dots is the same as that between the foci of the miscroscopes. 



The first question which arises in respect to the new apparatus is 

 this : Has the attempt to effect a perfect compensation been practi- 

 cally successful ? The answer, we apprehend, will scarcely be satisfac- 

 tory'. It was seen from the first that a disturbing cause exists in the 

 unequal facilities with which different metals receive and part with 

 heat, in consequence of which the two bars, at the time they were used 

 in the measurement, would probably have different temperatures ; in 

 which case the distance between the compensation j)oints would ne- 

 cessarily be altered, and the error arising from such alteration might 

 even exceed the whole amount of the expansion of a simple bar for 

 a corresponding variation of temperature. To remedy this inconve- 

 nience, the idea was suggested of covering the bars with coats of 

 varnish, so as to produce at least a similarity of surface. A great 

 number of experiments were made with a view to test the adequacy 

 of this remedy ; and as the results were considered to be satisfactorj% 

 the measuring bars, as well as the connecting bars of the microscopic 

 apparatus, were varnished accordingly. In the present measurement 

 it seems to be assumed that the disturbing cause was by this means 

 completely removed. Colonel Everest, on the contrary, found that 

 this was not by any means the case ; and he explicitly states that he 

 considered the advantages of the varnish to be more imaginary than 

 real. But it is manifest that if the compensation is only partially 

 effected, the method loses at once all its theoretical advantages ; and 

 is even attended with this disadvantage, that inasmuch as the effect 

 of a varying temperature on the compound l)ars is unknown, no 

 allowance can be made for it as in the case of a simple bar whose 

 rate of expansion is determined, and follows a known law. 



As the distance between the compensation points of a set of bars 

 cannot by any effort of art be made equal to 10 feet exactly, and as 

 the distance in res])ect of any two sets will not be exact/]/ the same, 

 each of the six sets used in the measurement, after the compensation 



