Metijurement of a Bafe on Hounflow-Heatlm 
*The manner in which the prifms are faftened to the bottoms of 
their refipeddive troughs, and the nature of the apparatufes they 
oarry on their extremities, will be readily conceived, by refer- 
ring to the particular plans and elevations of them, compre- 
hended in the group of eight fmall figures towards the right- 
hand of the general plan. Four of thefe appertain to the left- 
hand or fixed microfcope ; and the other four to the right-hand 
or micrometer microfcope, fo didinguifhed becaufe it has a 
micrometer attached to it. By means of the brafs collars 
which embrace the ptifms, their left-hand or fixed' ends are 
fcrewed down extremely fall: to the brafs pieces whereon they 
reft, fo as to be perfectly immoveable there with regard to 
their troughs ; whereas their right-hand ends are kept ealy, 
yet without fliake, in their collars, that they may contract or 
lengthen freely as the temperature may require, without occa- 
fioning any drain upon the parts. The prifm in the neared 
trough may be called the eye-piece prifm, becaufe it carries the 
eye-pieces of the microfcopes ; and that in the farther trough, 
the mark prifm, becaufe it carries the marks or crofs wires at 
which the microfcopes refpedtively point. The troughs are 
covered with pitch in the infide, to make them hold water ; 
and each has a cock in the left-hand end for difcharging it. 
Between the two deal troughs, one of copper, as a boiler, 
is placed, fomewhat fhorter than the former, but dill upwards 
of five feet in length. It is about 2f inches broad, and 3I in 
depth. The center of the boiler, or rather the center of the 
objedt lens which dands in it, as we lhall have occafion foon to 
point out, is didant from the crofs wires of the mark 5.81 
inches ; and from the wires of the micrometer attached to the 
correfponding eye-piece 20.33 i nc hes. The boiler reds on five 
fmall rollers, one being fixed to each end of the frame, and 
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