ORIGIN AND OPERATIONS 



collimation line; and the other, partial loss of light by reflection from the prism. As the col- 

 limating eye-piece and level may be made mutual checks, and theory gives us total reflection 

 from a prism, I am not satisfied that either are valid objections. To counterbalance these, if 

 actual demerits, the frequent uncertainty as to the amount of flexure and its positive source of 

 error, the risk from mounting a tall ladder to examine the nadir point, and of disturbing the 

 instrument when rising after the observation of an object near the zenith, are all in a great 

 measure avoided, whilst the labor of the observer is facilitated by having the micrometers next 

 him, and the accomplishment of his task in a uniform position, and that, the most perfectly 

 natural attitude of his body. Should the telescope have a micrometer at its eye-end for both 

 vertical and horizontal measurements, the uncertainty of semi-diameter and the effect of irra- 

 diation will be partially avoided, and a suitable screen can be constructed to shield it from solar 

 heat, as well as from that of the person. Are there other objections to such construction of 

 the instrument that I do not anticipate, and which would render it improper? 



"Secondly: I have not so great confidence in the results of micrometrical determinations, 

 except on occasions when, in consequence of the orbit of Venus lying near large stars, we may 

 be enabled to make comparisons by daylight. There will be two such periods at each conjunc- 

 tion of 1849 and of 1850-'51 ; and if we obtain simultaneous observations from the Cape of 

 Good Hope and Paramatta, as I doubt not we shall, a parallax in right ascension will be 

 measured at the same time. 



"If the government be pleased to authorize the Expedition, be assured I would not even 

 undertake its equipment without personal consultation with one whose ability and judgment 

 command my admiration as warmly as his personal merits entitle him to my esteem." 



On the fall January, 1848, a committee of the American Philosophical Society, consisting of 

 Profs. K. M. Patterson, K. S. McCulloh, and E. Otis Kendall, reported as follows : " That the 

 method for determining more accurately the dimensions of the solar system by similar observa- 

 tions of Venus in the northern and southern hemispheres, at the conjunctions of that planet, 

 proposed by Dr. Gerling in his letter to Lieut. Gilliss of April I 1 ?, 1847, is in their opinion 

 practicable, and therefore worthy of attention and patronage. Also, that the plan of Lieut. 

 Gilliss for carrying the views of Dr. Gerling into effect is well conceived, and if successfully 

 accomplished, cannot fail to confer honor on our country and its naval service. 



" They respectfully remind the society that, in the year 1769, observations of the transit of 

 Venus, for the determination of this important astronomical problem, were made by Dr. Rit- 

 tenhouse and others, under the liberal patronage of the government of Pennsylvania, then a 

 British colony, which observations were of great value, and justly reflected much honor upon 

 the colonial government, and particularly upon the distinguished men who were engaged in 

 making them. As the subject is, therefore, one which is already connected with the history of 

 American science, under the auspices of government patronage, as our country has hitherto 

 contributed but little, comparatively with the other principal nations, to astronomy and navi- 

 gation, and as the plan of Lieut. Gilliss is so truly American, your committee suggest that the 

 society should commend it earnestly to t.he attention and patronage of the Navy Department ; 

 or, if that department should not possess the legal authority to carry it into effect, to the favor- 

 able action of Congress. And they therefore propose the following resolutions : 



"Besolved, That the proposed method of Dr. Gerling, of Marburg, for determining the solar 

 parallax by observations of the planet Venus, when stationary and at the conjunctions, and 

 the plan of Lieut. Gilliss for its accomplishment by means of similar observations at the Naval 

 Observatory at Washington and in South America, would, if successfully carried out under the 

 direction of the Navy Department, furnish valuable astronomical data, and confer honor upon 

 our country. 



"Resolved, therefore, That this society do hereby commend the proposed plan to the favor 

 and adoption of the present distinguished head of that department; or, if necessary, to the Con- 



