VOL. LV.] PHILOSOPHICAL TRANSACTIONS. 197 



It may be doubted whether the mineral alkali be not generated on the peak, 

 where it is found, by the fitness of that sort of earth to attract out of the air 

 some of the principles of which it is made: for there is often seen on walls a sa- 

 line efflorescence, which proves to be this very salt: and some earth, as that at 

 the bottom of a lake in Egypt, is said to produce it, so as to make a constant 

 supply of a great quantity, which is every year dug up and carried away. The 

 natron must be in great abundance in the air or earth, as it is the base of that 

 salt which is the commonest of all, in almost every part of the world; but though 

 it be every where found when united to the acid of sea-salt, yet there are but 

 very few places where we have been able to procure it by itself. 



IX. Short and Easy Methods for finding, 1 . The Quantity of Time contained in 

 any given Number of Mean Lunations ; 2. The Number of Mean Lunations 

 contained in any given Quantity of Time; 3. The Number of Troy Pounds 

 contained in any given Number of Avoirdupois Pounds, and vice vtrsd; 4. The 

 Quantity and Weight of JVater contained in a full Pipe of any given Height, 

 and Diameter of Bore; and consequ£ntly, to find what Degree of Power would 

 be required to work a common Pump, or any other Hydraulic Engine, when the 

 Diameter of the Pump bore, and the Height to which the Water is to be raised 

 in it, are given. By Mr. James Ferguson, F.R.S. p. 6l. 



These methods may all be found reprinted and arranged, among many other 

 useful rules, in the author's volume of Tables and Tracts, published in 17 67. j 



X. A Recommendation of Hadleys Quadrant for Surveying, especially the Sur- 

 veying of Harbours; with a particular Aplication of it in some Cases of Pi- 

 lotage. By the Rev. John Mitchell, B. D., F. R. S. p. 70. 



The use of Hadley's quadrant, as an instrument to take altitudes at sea, is al- 

 ready so well established, that it wants no further recommendation. But there 

 are several other purposes to which it may be applied, with great advantage, 

 which, though obvious enough, seem yet to be hardly sufficiently attended to. 

 There is no instrument so well adapted to many kinds of surveying, either for 

 exactness or conveniency, and particularly the last; but the surveying of han. 

 hours, or such sands as lie within sight of land, may often be performed by it, 

 not only with vastly more ease, but also with a much greater degree of precision, 

 than can be hoped for by any other means, as it is the only instrument in use, in 

 which neither the exactness of the observations, nor the ease with which they 

 may be taken, are sensibly affected by the motion of a vessel : and hence a single 

 observer in a boat, may generally determine the situation of any place he pleases, 

 with a sufficient degree of accuracy, if, with this instrument, he takes the angles 

 subtended by 2 or 3 pairs of objects, properly chosen on the shores round about 



