Meafurement of a B'afe o« Hounllow-Heath. 46^ 



The manner in which the prifms are fastened to the bottoms of 

 their refpe6live troughs, and the nature of the apparatufes they 

 carry 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 fmaU 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 diftinguiflied becaufe it has a 

 micrometer attached to it. By means of the brafs collars 

 which embrace the prifms, their left-hand or fixed ends are 

 fcrewed down extremely fafl to the brafs pieces whereon they 

 reft, fo as to be perfe6lly immoveable there with regard to 

 their troughs ; whereas their right-hand ends are kept ealy, 

 yet without fhake, in their collars, that they may contract or 

 lengthen freely as the temperature may require, without occa- 

 fioning any ftrain upon the parts. The prifm in the neareft 

 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 refpedlively points 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 flill upwards 

 of five feet in length. It is about 2| inches broad, and 3I in 

 depth. The center of the boiler, or rather the center of the 

 objeft lens which ftands ia it, as we fhall have occaiion fbon to 

 point out, is diftant 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 inches. The boiler refls on five 

 fmall rollers, one being fixed to each end of the frame, and 



Vol. LXXV. P p p the 



