His baptism index:

First name(s) Robert
Last name Longman
Birth year 1813
Baptism year 1813
Baptism date 21 Jun 1813
Baptism place Sherborne
Parents' name(s) Son of John & Sarah
Father's first name(s) -
Mother's first name(s) -
Notes Druggist
County Dorset
Country England
Record set Dorset Baptisms
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A marriage or two:

First name(s) ROBERT
Last name LONGMAN
Marriage quarter 3
Marriage year 1840
Registration month -
MarriageFinder™ ROBERT LONGMAN married one of these people
SUSAN COOMBS, SARAH LONGMAN, SUSAN RIDOUT, ANN HUNT
District SHERBORNE
District number -
County Dorset
Country England
Volume 8
Page 160
Record set England & Wales marriages 1837-2008


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In the 1851 census a Robert in Sherborne was married to a Susan and was a Chemist,  Shades of his father’s profession of Druggist, as shown on the baptisms of his infants:

First name(s) Last name Relationship Marital status  Gender  Age  Birth year  Occupation  Birth place

Robert Longman Head Married Male 37 1814 Chemist Sherborne, Dorset
Susan Longman Wife Married Female 42 1809 - Sherborne, Dorset
Fanny Fool Servant Unmarried Female 25 1826 House Servant  South Cadbury, Somerset

Census ref:  Registration district Sherborne
Archive reference HO107
Piece number 1859
Folio 201
Page 36

And in 1861, there was a Married actuary, but no wife:

First name(s)  Last name Relationship Marital status  Gender  Age  Birth year  Occupation  Birth place

Robert Longman Head ... Male 47 1814 ... Sherborne, Dorset
Mary G Banger Servant Unmarried Female 23 1838  Domestic Servant  Longburton, Dorset

And in 1871, Susan was back in residence and he was a retired chemist:

First name(s) Last name Relationship Marital status  Gender  Age  Birth year  Occupation  Birth place

Robert Longman Head - Male 57 1814 - Dorset
Susan Longman Wife - Female 63 1808 - Dorset
Mary Ann Penny Servant - Female 22 1849 - Somerset

Similarly in 1881 and and in 1891 he was a widower.
________________

Either of these deaths, particularly the third in Sherborne, could be his, but needs a census check or two:

First name(s) ROBERT
Last name LONGMAN
Gender Male
Birth day -
Birth month -
Birth year 1814
Age 67
Death quarter 3
Death year 1881
District DORCHESTER
County Dorset
Volume 5A
Page 183
Country England
Record set England & Wales deaths 1837-2007

First name(s) ROBERT
Last name LONGMAN
Gender Male
Birth day -
Birth month -
Birth year 1813
Age 74
Death quarter 1
Death year 1887
District DORCHESTER
County Dorset
Volume 5A
Page 232
Country England
Record set England & Wales deaths 1837-2007

First name(s) ROBERT
Last name LONGMAN
Gender Male
Birth day -
Birth month -
Birth year 1813
Age 81
Death quarter 3
Death year 1894
District SHERBORNE
County Dorset
Volume 5A
Page 178
Country England
Record set England & Wales deaths 1837-2007
________________

I can see no sign of any children, so he is DSP>