Philosophical TVansacfions, >301 



certain the quantities of gas consumed by each of the burner? in 

 the same time ; multiply these quantities by the squares of the 

 respective distances, and the product will be the relative quanti- 

 ties of light, afforded by the gases. Let d he the distance of the 

 coal gas light, and d' that of the oil gas light ; and let q be the 

 quantity of coal gas consumed in a given time, and q the quantity 

 of oil gas consumed in the same time, then the intensity of the 

 coal gas will be to that of the oil gas q (P to q' d"^." 



The following paragraph we do not clearly understand. 



*' To find the ratio between the quantities of light given out by 

 the sun, and that afforded by a common candle, place one end of 

 the instrument opposite the sun, and bring the candle opposite the 

 other end, till the fluid in the stem remain stationary at the ori- 

 ginal division, and the light given out by the candle, will evi- 

 dently be to that given out by the sun, as the square of a few 

 inches to the square of the number of inches contained in 

 95,000,000 miles, provided none of the sun's light had been ab- 

 sorbed in its passage through the atmosphere." 



If this instrument really possess the qualities ascribed to it by 

 its author, it is well calculated for many important investigations 

 to which neither Mr. Leslie's photometer, nor any similar gim- 

 cracks, are at all applicable ; but we are somewhat apprehensive 

 that Mr. Ritchie has over-valued the accuracy of its indications, 

 as w^ell as the facility of its employment. If not, we hope soon to 

 hear more of it. 



vW. Description of a Floating Collimator, ^y Captain Henry ! 

 Kater, F. R. S. 



The apparatus described in this paper is intended to determine the 

 situation of the line of collimation of a telescope attached to an as- 

 tronomical circle, with respect to the zenith or horizon, in some one 

 position of the instrument. — In other words, to determine the zero 

 point of the divisions on the limb. This is at present usually per- 

 formed by the use of the level or the plumb-line, or by the re- 

 flection of an object from the surface of a fluid. The author de- 

 scribes the defects and inconveniencies of each of these methocjs. 

 Those of the plumb-line, when applied to small instruments, (ta 

 the improvement of which he describes his attention to have been 

 particularly directed,) are referrible chiefly to want of sufficient 

 delicacy. Those of the level are referrible to a variety of causes 

 not under the command of the observer. While observations by 

 reflection, the most perfect perhaps of any now practised, require 

 a union of favourable circumstances rarely occurring. Add to these, 

 when levels or plumb-lines are used, the necessity of reversing the 

 instruments — and obsei-ving out of the meridian — and when obser- 

 vations are made by reflection that of deferring the corresponding 



