generally been placed upon it. When instances were quoted of elevations determined by the
theodolite and the barometer agreeing to within a few feet of each other, as illustrations of the
length to which accuracy might be carried, there might in reality be very little merit due
to any one-the coincidence being matter of accident, and in not one experiment out of
ten would the same results be attained by the same observer, making use of the same instruments
under circumstances as nearly as possible identical. The error might not be very great, but it
was well to know that it was impossible to provide against the elimination of error altogether.
The experiment bore on another point : in the examination of that very ill understood phenome.
non, the daily fluctuation of the barometer, it was considered matter of much importance
to know at what rate the amount of tide decreased as we assended from the surface of the
earth. The best experiments on this subject were those made simultaneous by Colonel Sykes at
Poona and Dr Walker at Mahabuleshwar-the difference of level being 2500 feet; with those
made at Madras, twenty feet, Trevandrum, 150 feet, and Dodabetta, 8500 feet, above the level
of the sea, and those made at Aden by Mr Mayes on the Seerah Island, 180 feet, and on
the top of Shum Shum, 1700 feet above the level of the sea. In this last case the tide at
the higher point ought to be about one twentieth less than that of the lower: at Dodabetta,
it ought to be one third less than at Madras, and less they were accordingly, in somewhat lik
these proportions; but until either better instruments than the best of our barometers were
procured, or a much larger mass of observations than that now existing were accumulated,
it would be unsafe positively to lay down the law on the subject. No sufficient explanation
of these discrepancies had been given: Dr Ford ascribed them to the electric state of the baro-
metric tube, to which very probably might be due. In want of explanation of their cause,
it was
well to be in possession of the fact of their existence. (See next page.)

The Aneroid is an instrument of so much value for survey purposes in India, that I have lost no opportunity of endeavouring to ascertain its trustworthiness, and in general the results of these inquiries have redounded greatly to its praise. An Aneroid appears to have been kept at the Colabah Observatory for the past twelve months, comparative readings of which with the Standard Barometer have been published in the Newspapers.

From this it will

The following is an abstract of these for the whole of the year 1850. be seen that the range of the Aneroid which at the commencement of the year was close on that of the Barometer has to wards conclusion been reduced to very nearly a fourth of that instrument. The standard Barometers at the Observatory are the finest instruments that can be made, and there seems no reason to doubt the perfect care and accuracy with which they are observed. Of the Aneroid in use I cannot speak, but it is observed, I presume by the same parties who read the Barometer, and has therefore ikely to be treated with every justice. If under these circumstances the Aneroid is apt to deteriorate so fearfully, our faith in it must be materially modified, and those instruments only trusted which have before hand been subjected to long and careful examination. (See page xxxiii).

STANDARD MOUNTAIN BAROMETERS.

Barometers, Comparison of, made at Colaba, on the 5th and 6th February 1851. Elevation above the sea 37 feet.

Mean Atd. No.

Time. Ther. 9 M.

No.
11

No. No. No. No. No. No. No. No. No.

12

16 17 18

9

9

3

8

9

1234

⚫080 076 ⚫081 ⚫080 ⚫070 ⚫076 •C86 ⚫080 ⚫096 ⚫065
⚫066 074 ⚫010 ⚫978
77-0 ⚫080 *076 *070 ⚫080 •066 .074 ⚫074 ⚫076 -078 •060 ⚫060 ⚫074 •010 -977
77.0 ⚫076 -070 '062 ⚫074 ⚫061 ⚫063 ⚫070 ⚫072 *065 ⚫058 *060
⚫070 ⚫005
76.8 *054 060 *956 ⚫064 ⚫050 ⚫058 ⚫065 ⚫070 *060 ⚫050 ⚫053 ⚫058 29 990
76.5 '054 .050 *048 ⚫052 -843 ⚫050 ⚫060 ⚫077 *054 -046 ⚫058
⚫042 30-000

•100 *086 *092 *092 *088
•130 116 •112
8 78.8 •120 •123 124 •126 .116 .116 112 •150 •136 •126
80.8 •146 .156 .146 .148 •138 -144 •140 •156 158 •164
10 82.0 •146 •156 152 •152 .136 •150 •140 •170 •160 •144 •134 ⚫154
11 83.5 •125 •130 *125 ⚫120 .112 .118 •124 .126 •126 116
Noon 84-0 ⚫086 ⚫075 *076 ⚫093 ⚫070
-076 *078 -092 *092 ⚫078
⚫090 ⚫020 -975 29.980 ⚫940 '020 ·02 29.97
1 84-0 ⚫046 ⚫035 *044 040 ⚫036 .032
*042 ·058 *050 ⚫044 *044
⚫056 29.975
83.8 *020
⚫008 •010 -020 ⚫006 -000 ⚫016 ⚫036 ⚫028 -016 ⚫016 ⚫030 *955 •950 *925
82.5 •OLO 004 29 990 29-988 29.984 29-992 *002 ⚫036 ⚫020 ⚫000 .000 ⚫016 •960 ⚫940 ⚫920
82-0 ⚫014 010 30 000 30 018 30-006 -992 ⚫012 ⚫046 ⚫032
⚫028 ⚫022 *024 •950 .950
•950
81.0 ⚫028
⚫024 ⚫032 ⚫026 *028 .036 ⚫030 ⚫050 ⚫056 *048 ⚫030 ⚫030 .970 •950 *955 .900
80.5 ⚫050 ⚫054 ⚫048
⚫060 ⚫042 .052 *052 -070 ⚫064 ⚫070 ⚫040 ⚫060 •990 .960 +965
80.5 ⚫058 -072 ⚫060 ⚫068 ⚫052 ⚫060 ⚫060
⚫080 ⚫070 ⚫076 ⚫072
.975 •970
80.0 ⚫086 *082 *080 *082 ⚫074 ⚫092 *076 •114 •100 ⚫096 ⚫090
80.0 ⚫094 •100 ⚫096 ⚫090 ⚫084 -100 •100 •128 ⚫124 •106 -092
10 79.0 ⚫088 •112 ⚫098 *088 ⚫084 ⚫096 •100 .108 •104 •102 ⚫083 ⚫084
11 78.0 *088 •100 •106 ⚫080 ⚫080 ⚫076 -086 -084 ⚫096 ⚫082 -078
Mid-
Night 77.5

13

14 15

19

in. in. in. in. in. in. in. in. in. in. in. in. in. in. in. in. in. in. in.

.960 30.005

78-5 30-044 30-066 30.060 30-060 30-054 30-054 30.050 30-086 30-072 30-060 30-054 30-070 30.000 29-978 29.980 29 950 29-995 30-04
3 77.5 ⚫078 *074 063 *070 ⚫072 ⚫070 ⚫064 ⚫096 •100 ⚫094 ⚫080 *084 •011 •975 ⚫990
77-5 ⚫080 ⚫086 ·080 *076 *082 ⚫086 *076 •110 .114 •106 ⚫088 *096 ⚫020 •985 30-015 .975
7 76.7 ⚫090 ∙104
•100 •106 ⚫025 ⚫990 ⚫020 .990

in.

Bar.

20

No. No. No. No.
4406 4415 4521 4525

No. No. No. No. No. 4959 2736

No.

2738

2739 2741

2691 M.

in.

in. in.

Means.

79-8 20 047 30-075 30-073 30-074 30 065 30-071 30-072 30-088 30-084 30-077 30-071 30-078 30 012 29 975 29-982 29-965 30-000 30-03 29.99 30-05 30-08 30-03

Dr Buist laid two other papers before the Society-one on the decrease of the range of the Observatory Aneroid, which in January 1850 had not differed more than 020 from the standard barometer, and which now ranged about 040 when the standard ranged 130. If the instrument was apt to deteriorate at this rate, it could only be relied on where the means of comparing it from time to time with the barometer were at command. The other paper was on the hourly increments and decrements of pressure at some half score of places betwixt the Tropics.

BAROMETER AND ANEROID COMPARISON, 1850.

The Aneroid is an instrument of so much value for survey purposes in India, that I have lost no opportunity of endeavouring to ascertain its trustworthiness, and in general the results of these inquiries have redounded greatly to its praise. An Aneroid appears to have been kept at the Colabah Observatory for the past twelve months, comparative readings of which with the Standard Barometer have been published in the Newspapers.

The following is an abstract of these for the whole of the year 1850. From this it will be seen that the range of the Aneroid which at the commencement of the year was close on that of the Barometer, has towards conclusion been reduced to very nearly a fourth of that instrument. The standard Barometers at the Observatory are the finest instruments that can be made, and there seems no reason to doubt the perfect care and accuracy with which they are observed. Of the Aneroid in use I cannot speak, but it is observed, I presume by the same parties who read the Barometer, and has therefore likely to be treated with every justice. If under these circumstances the Aneroid is apt to deteriorate so fearfully, our faith in it must be materially modified, and those instruments only trusted which have before hand been subjected to long and careful examination. (See page xxxiii).

Comparison betwixt the Standard and Aneroid Barometers made at the Observatory Colaba, betwixt January and December 1850, with the range of each; showing the remarkable diminution of the Aneroid in the course of the year.