GLEANINGS IN SCIENCE. 
27 
This instrument is by far the most generally useful we have seen of its size, 
and we have no hesitation in offering our recommendation to any one who may 
contemplate the possession of an instrument of this price to order one from Mr. 
Dolland. The circles are 12 inches diameter. It is contained in two boxes, of 
moderate size, and quite portable. 
Every part of it except the divisions is covered with paint, which we think an 
improvement. The price we understand was £113, and in this country, including 
charges, Rs. 1230. 
3. Lunar Distance*. 
It is well known that the great desiderata in physical geography are good dif- 
ferences of longitude, those of latitude being easily obtainable, and with no great ap- 
paratus, nor involving much trouble. Of the several methods of obtaining the for- 
mer generally recommended, there is but one that does not require expensive in- 
struments, and involve the loss of much time, if not troublesome calculation : — that 
is, the method by transference of time. Even this method requires at least two 
chronometers and an instrument for taking the time, while if the difference of 
longitude exceed a certain quantity, they require several repetitions of the opera- 
tion, and consequently time. The method I am about to recommend may be used 
by a solitary observer, whatever the difference of longitude. It requires hut a 
Troughton’s circle and sextant, with a tolerable pocket watch. He may choose his 
own time for the operations, nor is the calculation (in the present day) such as to 
form any valid objection. 
The method I would propose is that of Lunar Distances, which I am not aware of 
having ever seen recommended for this purpose*. When absolute longitudes are de- 
termined in this way, the result is vitiated by the error of the tables, or calculated 
place of the moon. But as it is probable that this error, whatever it may be, has 
a uniform effect within a certain interval of time, it is likely that though two lon- 
gitudes determined under this condition would have nearly the same error, yet 
that the difference of longitude would not be very far removed from the truth. 
Thus, to give an example. 
The longitude of Parsauntpur was found by the lunar method (Troughton’s 
circle) to be 
H. M. S. 
5 38 27 9 
Of Loten village, 5 40 40.3 
Miles. 
Difference, 2 12.4 34 14 
By protraction, 30*01 
Error, 4.13 
Now as however great the difference of longitude, the probability is that the 
error would he much the same, we see here a method by which an unassisted in- 
dividual might greatly improve the geography of a country. And if it should so 
happen that a second and a third individual were similarly employed about the 
same time, it is obvious that their differences of longitude would he still more cor- 
rect. To a traveller, who is generally restricted as to time, I consider this method 
far preferable either to Lunar Transits or Satellites: for besides that no opportunity 
may offer with them, there is the great incumbrance of the instruments, — a serious 
objection, whichever way it be considered. Nor is it possible to set up a transit 
satisfactorily on any such temporary stage as a traveller would be likely to have at 
his command. In the method I jiropose, a Troughton’s circle, artificial horizon, and 
good pocket watch are all that Is required : — articles sufficiently portable, not in- 
volving much expense of outlay, or liable to serious injury in moving. The 
reductions have been so simplified latterly, as to form no objection to any one con- 
versant with astronomical calculation. To sum up, I am of opinion it is the most 
extensively available to the purposes of general geography of any method yet pro- 
posed, while the instruments required are such as no geographical traveller will be 
unprovided with— such in fact as, and no more than, are requisite for the determi- 
nation of the latitude. This in itself is a strong recommendation of the method. 
Its value would be greatly enhanced, were regular, daily distances observed by 
some fixed observer in any part of India. D. 7 
* 1 mean, for differences of longitude. 
