Royal Society* 305 



It being assumed that 100 degrees is the temperature of steam 

 when its pressure is in equilibrium with a column of 30 inches of 

 mercury, or with the pressure of one atmosphere, then F being the 

 pressure in atmospheres, at any temperature t, 



VI 00/ 



A comparison is instituted between theoretic experiments of the 

 Academy of Sciences and the results of this formula, from which it 

 appears that the temperatures deduced from the formula are inva- 

 riably in defect, the greatest difference being 3'bl, and the least 

 2-02. 



June 20. — " Observations on the Nebulae." By the Earl of Rosse, 

 Pres.R.S., &c. &c. 



The object of this paper is to lay before the Royal Society an ac- 

 count of the progress which has been made up to the present time 

 in the re-examination of Sir John Herschel's Catalogue of Nebulae 

 published in the Phil. Trans, for 1833. 



Before describing any of the interesting objects the peculiar fea- 

 tures of which the extraordinary powers of the telescope employed 

 for their examination has brought to our knowledge, the author 

 enters upon some details concerning the instrument itself. This 

 telescope, which for apertures and the consequent power it possesses 

 for the examination of faint details must for a considerable time, at 

 least, remain unrivalled, has a clear aperture of 6 feet, with a focal 

 length of 53 feet. It has hitherto been used as a Newtonian, but 

 by the easy application of a little additional apparatus it may be 

 conveniently worked as a Herschelian ; and the author thinks it not 

 improbable that, in the further examination of the objects of most 

 promise with the full light of the speculum undiminished by a second 

 reflexion, some additional features of interest will come out. 



The tube reposes at its lower end upon a very massive universal 

 joint of cast-iron, resting upon a pier of stonework buried in the 

 ground, and it is counterpoised so that it can be moved in polar 

 distance with great facility. The extreme range of the tube in right 

 ascension at the equator is one hour, but greater as the polar distance 

 diminishes. By a little subsidiary apparatus the movement of the 

 telescope can be rendered almost exactly equatorial ; but up to the 

 present time this apparatus has not been used, as without it, the 

 movement was found to be sufficiently equatorial for such measure- 

 ments as have been required. The whole mounting was planned 

 especially with a view of carrying on a regular system of sweeping, 

 but as yet the discovery of new nebulae has formed no part of the 

 systematic work of the observatory, the known objects which re- 

 quire examination being so numerous that hitherto the observers 

 have been fully occupied with them. 



A clock movement was part of the original design, but as yet the 

 telescope is not provided with one, and the want of it has not been 

 very much felt. 



Various micrometers have been tried, but, upon the whole, the 



Phil. Mag, S, 3, Vol. 37, No. 250. Oct, 1850. X 



