506 PHILOSOPHICAL TKANSACTIONS. [aNNO 1731. 



calculus in 1690 and 1708, for instance, dift'ered insensibly from what he found 

 in the like situation of the sun and apogaeum in 1726. The great agreement 

 of the theory with the heavens compensating the differences, that might otlier- 

 wise arise from the incommensurability and eccentricity of the motions of the 

 sun, moon and apogaeum. 



Encouraged by this event, the Dr. next examined what differences might 

 arise from the period of 9 years wanting 9 days; in which time there are per- 

 formed very nearly 1 I 1 lunations, or returns of the moon to the sun; but tfie 

 return of the sun to the apogjeum in that time differing above 4 tin)es as much 

 from an exact revolution, as in the period of 18 years, he could not expect the 

 like agreement in that. However, having now entered on the 10th year, he 

 compared what he had observed in 1721, 1722, with his late observations of 

 1730, 1731 ; and he rarely found a difference of more than one single minute 

 of motion, part of which may probably arise from the small uncertainty that 

 always attends astronomical observations, but most commonly this difference 

 was wholly insensible; so that by the help of what he observed in 1722, he 

 presumes he is able to compute the true place of the moon with certainty, 

 within the compass of 2 minutes of her motion during this present year 1731 ; 

 and so for the future. This is the exactness requisite to determine the longitude 

 at sea to 20 leagues under the equator, and to less than 15 leagues in the 

 British channel. 



It remains therefore to consider after what manner observations of the moon 

 may be made at sea with the same degree of exactness: but since the worthy 

 vice-president Mr. Hadley, to whom we are highly obliged for his having per- 

 fected and brought into common use the reflecting telescope, has been pleased 

 to communicate his most ingenious invention of an instrument for taking the 

 angles with great certainty by reflection, (vide Phil. Trans. N° 420) it is more 

 than probable that the same may be applied to taking angles at sea with the 

 desired accuracy. 



^n Account of the Contrayerva. By Dr. JVilliam Houston. N° 421, p. ipS. 



Contrayerva is a Spanish word, signifying as much as herba contra (venena) or 

 an herb against poisons. The name of contrayerva, seems to have been given 

 by the Spaniards to as many plants of this sort as have come under their know- 

 ledge. Dr. Houston, however, does not pretend to give a history of all those 

 roots, but only offers a short account of that plant, whose root is called con- 

 trayerva here in England, and well known to all that deal in medicines. 



The root itself being so commonly known, the Dr. confines himself to the 



