Prof. Barlow's description of a New Fluid Telescope. 93 



upper post limestone is very greatly superior in purity to the 

 limestone of the lower. Their extraneous ingredients are also 

 associated in very different proportions. But notwithstanding 

 this, a long experience has established the fact at the Clyde 

 Iron Works, that they have no mode of employing a flux so 

 efficacious, as by joining the two limestones in equal propor- 

 tions in the furnace. Neither limestone taken singly is found 

 so efficient as when both are joined in equal quantities. 



We have now completed the first part in our proposed me- 

 tallurgic treatise, and have considered the nature of ironstone, 

 fuel and fluxes, the three classes of crude materials that form 

 the elements, out of which it is the ironsmelter's art to produce 

 the most important of our metals. The methods of assaying 

 these crude materials, and the processes by which they are 

 prepared for the furnace, together with other particulars re- 

 lating to the iron manufacture, will form subjects of examina- 

 tion in the subsequent parts of this memoir. 



Art. XIV. — Description of the new Fluid Telescope re- 

 cently ccynstructed by Messrs W. and T. Gilbert on a 

 plan suggested hy Peter Barlow, Esq. F. R. S., Mem. 

 Imp. Ac. Petrop. In a Letter to the Editor. 



Dear Sir, 

 In reply to your letter I beg to forward for insertion in your 

 next Journal a description of my new telescope, which I hope 

 you will find intelligible. I have also added some of the tests 

 to which I have submitted it ; but as I have not yet had it at 

 Woolwich, and having been rather unfortunate on the nights 

 when I have been at Woodford, where it has been made un- 

 der the direction of Messrs W. and T. Gilbert, these are not 

 so numerous as I could wish. I hope, however, soon to have 

 it here, when I shall have better opportunities of trying its 

 power. A distance of twenty miles between an observer and 

 his instrument is no trifling impediment to astronomical ob- 

 servation, particularly at this season of the year, when the 

 state of the weather is so uncertain. 



